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Very overweight. Conventionally defined as a body mass index (weight/height squared) greater than 30 in humans.

Obligate aerobe / anaerobe

Aerobe = An organism that requires oxygen for cellular respiration and cannot live without it. Anaerobe = An organism that cannot use oxygen and is poisoned by it.


Recurrent, persistent ideas, thoughts, images, or impulses that are egodystonic, i.e., are not experienced as voluntarily produced, but rather as ideas that invade consciousness.  Obsessions are characteristic of obsessive compulsive disorder and may also be seen in schizophrenia.

Obtuse angle

An angle whose measure is greater than 90 degrees.

Obtuse triangle

A triangle with an obtuse angle (an angle whose measure is greater than 90 degrees.)

Occipital Lobe

Region in the back of the brain which processes visual information. Damage to this lobe can cause visual deficits.


The proportion of receptors to which a drug is bound. It may be calculated from the Hill-Langmuir adsorption isotherm:

 Occupancy  =  ___________
                K  +  [D]

where K is the dissociation constant for the drug and [D] is its concentration.

Occupational therapy

Occupational Therapy is the therapeutic use of self-care, work and play activities to increase independent function, enhance development and prevent disability; A therapy, treatment, or instructional support
provided by an occupational therapist to the child, family, and/or pertinent members of the child's environment.  It may include the adaptation of a task or the environment to achieve maximum independence and to enhance the quality of life. The term occupation, as used in occupational therapy, refers to any activity engaged in for evaluating, specifying and treating problems interfering with functional performance. It helps develop adaptive or physical skills that will aid in daily living and improve interactions with a person's physical and social world. It focuses on developing functional skills related to sensory-motor integration; coordination of movement; fine motor skills; self-help skills (dressing, self-feeding, etc.); adaptive devices/equipment; computer keyboarding; positioning for school work; and potential work-related activities.

Octal number

A number in base 8.

Ocular dominance columns

A pattern of organization in primary visual cortex in terms of the dominance of inputs from the two eyes to a vertical column of neurons.
Each ocular dominance column represents a vertical collection of neurons that have a dominant input from one eye. First described by Hubel and Wiesel in the primary visual cortex, ocular dominance is determined by the thalamic input to layer IV of the primary visual cortex.
(see  Orientation selective columns and Hypercolumns)

Odd function

A function that satisfies the property that f(-x) = -f(x).

Odd number

A whole number that is not divisible by 2.

Ohmic heating

A novel sterilization technique in which heat is generated within a food product due to its inherent resistance.

Ohmic ion channels

An ion channel through which ions flow equally well from one side of the membrane to the other. Ion channels are termed as ‘ohmic’ or ‘rectifier’ channels. In ohmic ion channels the ion flow is directly proportional to the voltage (I = Vm/R). In the second case the flow is not linearly related to the voltage as the ions flow more readily in one direction than the other. (see Ion channels or Rectifier ion channels)

Oligonucleotide ("oligo" means few)

A sequence of  ~15-75 nucleotides (DNA or RNA) that is chemically synthesised from the 3’- to 5’-end by a cyclical process adding one nucleotide at a time.
Short complementary DNA oligonucleotides are used as 3’- and 5’- primers in polymerase chain reaction (PCR).  Complementary RNA oligonucletotides (antisense oligonucleotides) are used to prevent translation of mRNA into protein. Labelled oligonucleotides are often used as probes to detect DNA (Southern blot) or RNA, as primers for DNA synthesis or
probes for arrays and ISH/FISH; usually referred to as "oligo(s)."


Low in nutrients.


A heterotrophic animal that consumes both meat and plant material.

Omissions (in articulation)

An articulation error that occurs when not all of the sounds in a word are articulated.
An articulation error that occurs when not all of the sounds in a word are articulated. This type of articulation problem is frequently described as infantile. Some sounds are omitted more frequently than others, and the position within a word can affect the presence or omission of a particular sound.


A mutated gene in a tumor virus or in the genome of cancerous cells which, when transferred into other cells, can cause or promote a cell to undergo unregulated growth and division (i.e., to undergo cancer).
Produced by mutations to proto-oncogenes. (see Proto-oncogene). Mutations in genes that become oncogenes can be inherited or caused by being exposed to substances in the environment that cause cancer.
Note that only certain cells are susceptible to transformation by any one oncogene.
Functional oncogenes are not present in normal cells. A normal cell has many "proto-oncogenes" which serve normal functions, and which under the right circumstances can be activated to become oncogenes. The prefix "v-" indicates that a gene is derived from a virus, and is generally an oncogene (like v-srcv-rasv-myb , etc).

One Standard Test

A one standard test is an immunoassay test in which only one standard is assayed.
This baseline standard, called the index standard, is used to calculate the ratio of each sample to the index standard response. The index standard's adjusted response can be either the numerator or the denominator to the sample's adjusted response. The ratio is then multiplied by the concentration entered for the index standard.


In medicine, the first appearance of the signs or symptoms of an illness as, for example, the onset of rheumatoid arthritis . There is always an onset to a disease but never to the return to good health. The default setting is good health.


The embryonic development of an organism.
The developmental history of an individual organism from its origin to its death.


Collections of statements that define the relations between concepts and specify logical rules for reasoning about them.


A cell that gives rise by meiosis to an ovum.


A condition in which male and female gametes differ, such that a small, flagellated sperm fertilizes a large, nonmotile egg.


The process in the ovary that results in the production of female gametes.

Open circulatory system

An arrangement of internal transport in which blood bathes the organs directly and there is no distinction between blood and interstitial fluid.

Open interval

An interval that does not contain both its endpoints.

Open-loop (or volitional) movements

Those movements triggered by a sensory cue or some internal desire to move.
See Closed-loop (or reflex) movements

Open reading frame

Any region of DNA or RNA where a protein could be encoded.
In other words, there must be a string of nucleotides (possibly starting with a Met codon) in which one of the three reading frames has no stop codons

Operant conditioning

A type of associative learning that directly affects behavior in a natural context; also called trial-and-error learning.

Operating system

The special software required to make a computer work.
It provides the link between the user and the hardware. Popular operating systems include: DOS, MacOS, VMS, VM, MVS, UNIX, and OS/2. (Note that "Windows 3.x" is not an operating system as such, since in must have DOS to work. )

Operator (in DNA)

A segment of DNA that interacts with a repressor protein to regulate the transcription of the structural genes of an operon .
The site at one end of an operon where a repressor molecule binds to the DNA and thereby inhibits transcription.
The term is appropriate also for the corresponding site when a gene does not form part of an operon.


A group of closely-linked genes coding for related proteins, and which appear to affect different steps in a single biosynthetic pathway and which apear to function as an integrated unit.
A unit of genetic function common in bacteria and phages, consisting of coordinately regulated clusters of genes with related functions.
An operon consists of an operator, promoter, regulator, and structural genes. The regulator gene codes for a repressor protein that binds to the operator, obstructing the promoter (thus, transcription) of the structural genes. The regulator does not have to be adjacent to other genes in the operon. If the repressor protein is removed, transcription may occur. Example: the Lac operon codes for b-galactosidase, for b-galactoside permease and for b-galactoside transacetylase.
The genes of an operon are transcribed in one messenger RNA molecule and thus an operon could be defined as an aggregate of genes with a single messenger.
Operons are either inducible or repressible according to the control mechanism. Both repression and induction are examples of negative control since the repressor proteins turn off transcription.


A form of spasm in which the head and the heels are bent backward and the body bowed forward.

Opportunistic infection

An infection caused by germs that are not usually capable of causing infection in normal people, but can do so given certain changes (opportunity) in the immune system.

Opportunistic species

Species characterized by high reproduction rates, rapid development, early reproduction, small body size, and uncertain adult survival.

Oppositional disorder

The covert display of underlying aggression by patterns of obstinate, but generally passive behavior.
Children with this disorder often provoke adults or other children by the use of negativism, stubbornness, dawdling, procrastination, and other behaviors.


An immune response in which the binding of antibodies to the surface of a microbe facilitates phagocytosis of the microbe by a macrophage.

Optic nerve

either of a pair of second cranial nerves; transmits visual data from retina to brain.

Optical frequency comb

A precision tool for measuring frequencies of light; the frequency spectrum of a stabilized mode-locked laser consisting of hundreds of thousands of sharp lines of different colors (frequencies) that resemble the teeth of a comb.

Optical frequency synthesis

Control of the frequency comb produced with a mode-locked laser to produce a desired optical frequency.

Optical pulse synthesis

The use of two phase-locked-mode-locked femtosecond lasers to produce a new, coherent pulse with larger amplitude than the individual laser outputs.

Optical trap

A tightly focused beam of light capable of holding particles stable in three dimensions; optical tweezers.

Optical tweezers

A focused laser beam that can manipulate micron-sized objects in solution.

Optoelectronic device

An instrument that is or uses a electrical-to-optical or optical-to-electrical transducer.


An eye doctor. A physician practicing ophthalmology. An ophthalmologist is an M.D.


A painful inflammation of the testicles and can lead to hypogonadism.

Oral hypersensitivity

Where touch to the mouth is unpleasant.
This occurs when there is no oral stimulation and the mouth changes in ways that touch that used to be pleasurable, is now unpleasurable. The mouth becomes unfamiliar to taste, touch, texture, and other stimuli. An oral motor program is often begun by an occupational therapist and continued at home by the caregiver.

Oral Polio Vaccine (OPV)

An immunization that protects children against polio.
Recommended administration is at 2 months, 4 months, 15 months, and 4-6 years of age


In the current model of atomic structure, the volume of space surrounding the atomic nucleus in which an electron will be found 90 percent of the time.


A taxonomic grouping of related, similar families; the category below class and above family.An intermediate level of taxonomic classification containing one or more Families forming a subgroup of a Class.

Ordered pair

A set of two numbers in which the order has an agreed upon meaning.  Such as the cartesian coordinates (x, y), where it is agreed that the first coordinate represents the horizontal position, and the second coordinate represents the vertical position.


The second coordinate of a cartesian ordered pair.


Describes something that stimulates the intake of food. Hence anorexigenic = inhibits food intake.


A specialized center of body function composed of several different types of tissues.


Pertaining to, or derived from, biological material.

Organ-identity gene

A plant gene in which a mutation causes a floral organ to develop in the wrong location.


Any living entity.

Organ of Corti

A structure within the inner ear (the cochlea) of the vertebrate ear.
It is made up of a membrane running the length of the cochlea, on which sit a number of cells principal of which are contains the receptor cells (hair cells) that are ultimately responsible for converting the pressure waves of sound into a biological response (a change in the electrical properties of the receptor cells).


One of several subcellular formed bodies in eukaryotic cells, with a specialized function, suspended in the cytoplasm. Membrane-bound structures in a eukaryotic cell that partitions the cell into regions which carry out different cellular functions, e.g., mitochondriaendoplasmic reticulumlysosomes.


Pertaining to (1) organisms or living things generally, or (2) compounds formed by living organisms, or (3) the chemistry of compounds containing carbon.
Compounds that contain carbon, such as vitamins, carbohydrates, proteins and fats, but not minerals. Organic is a chemical term designating compounds containing a carbon skeleton plus hydrogens, oxygen, and in smaller and variable amounts nitrogen, phosphorus, or sulfur.

Organic chemistry

The study of carbon compounds (organic compounds).

Organic compound

A chemical comound containing the element carbon and usually synthesized by cells.

Organic matter

plant and animal residues, or substances made by living organisms. All are based upon carbon compounds.


An individual living thing, such as a plant, animal or micro-organism (bacterium, fungus, protest). The individual member of a species; can be a single cell or a multicellular organism. Organisms are the biological unit of reproduction and while cells of single cell organism are autonomous (bacteria, archaea), individual cells of multicellular organisms (fungi, plants, animals) are not.


An early period of rapid embryonic development in which the organs take form from the primary germ layers.


Subjective impressions obtained using the human senses.


Rhythmic, involuntary contractions of certain reproductive structures in both sexes during the human sexual response cycle.


Awareness of one's environment and/or situation, along with the ability to use this information appropriately in a functional setting.

Orientation selective columns

A pattern of organization in primary visual cortex in terms of the preferred stimulus orientation to a vertical column of neurons.
Each orientation selective column represents a vertical collection of neurons that have a preferred orientation for a visual stimulus. First described by Hubel and Wiesel in the primary visual cortex.
(see  Ocular dominance columns and Hypercolumns)


In anatomy: The attachment of muscle (by means of a tendon) to the stationary bone.
In mathematics: The point (0, 0) on a Cartesian Coordinate System.
Normally the end that is stationary when the muscle contracts. (See Insertion). The term has been replaced by the newer term Attachment.

Origin of replication

A specific sequence of bases in a nucleic acid molecule to which the enzymes responsible for replicating the nucleic acid bind to initiate the copying process.
Nucleotide sequences present in a plasmid which are necessary for that plasmid to replicate in the bacterial host. Eukaryotic genomes are replicated bidirectionally from multiple replication origins. To activate an origin, the origin recognition complex loads several factors, including the hexameric helicase Mcm2-7. The Mcm2-7 proteins are loaded cooperatively, leading to mobile, head-to-head double hexamers of Mcm2-7.

Oropharyngeal airway (see also Nasopharyngeal airway)

An artificial airway that is inserted into the mouth and separates the tongue from the posterior pharynx


The point in a triangle where the three altitudes intersect.




The term ortholog is used to indicate an evolutionary related gene existing in two or more different organism. Orthologous genes have a high degree of similarity or sequence identity (see similarity). Orthology is a important way of assessing an organisms evolutionary history. For instance, some two thirds or all human genes have orthologs in the fruit fly Drosophila melanogaster. Humans share 99% of their genes with chimpanzee. Thus the degree of orthology correlates with the evolutionary relatedness between organisms (see also paralogs).


The branch of medicine devoted to the study and treatment of the skeletal system, its joints, muscles and associated structures. The branch of surgery broadly concerned with the skeletal system (bones). Orthopedics is how this field of surgery is listed under Physicians & Surgeons in the telephone Yellow Pages in Jacksonville, Florida. This spelling is quite common today. But it is incorrect, erroneous, flat out wrong. Orthopedics would relate the term to the foot because in Latin pedis means foot. Orthopaedics is not merely old-fashioned. It is the correct spelling. What was meant by the term orthopaedics when it was devised goes back to its roots: ortho-, straight + the Greek paes, child = the practice, literally, of straightening the child. If the child had a crooked spine ( scoliosis ), it was the job of the orthopaedist to straighten the child, not just the child's foot. That is why there is no American Academy of Orthopedic Surgery but there is an American Academy of Orthopaedic Surgery. And that is also why orthopaedists look after broken bones.


A condition in which a person is able to breathe most comfortably only in the upright position


Splint or brace designed to improve function or provide stability. An orthopedic appliance used to support, align, prevent or correct deformities or to improve the functioning of movable parts.

Oscillating magnetic fields

Magnetic fields generated with electromagnets of alternating current. The intensity varies periodically according to the frequency and type of wave in the magnet.


An animal that does not actively adjust its internal osmolarity because it is isotonic with its environment.


Adaptations to control the water balance in organisms living in hypertonichypotonic, or terrestrial environments.


An animal whose body fluids have a different osmolarity than the environment. 
It must either discharge excess water if it lives in a hypotonic environment or take in water if it inhabits a hypertonic environment.


Tendency for fluids to mix, or become equally diffused, when in contact.
It can be observed between fluids of differing densities. Osmose may be regarde as a form of molecular attraction, allied to that of adhesion.


The diffusion of water from a region of low solute concentration to one of high solute concentration, through a selectively-permeable membrane. Diffusion of water across a semi-permeable barrier such as a cell membrane, from high water potential (concentration) (i.e.,a region of low-solute concentration) to lower water potential (concentration) (i.e., a region of low-water/high-solute concentration).

Osmotic potential

The tendency of water to move across a selectively permeable membrane into a solution. It is determined by measuring the pressure required to stop the osmotic movement of water into the solution.

Osmotic pressure

A measure of the tendency of a solution to take up water when separated from pure water by a selectively permeable membrane.
The hydrostatic pressure generated by a solution moving by osmosis across a selectively-permeable membrane (e.g., into or out of a cell) because of differences in concentration of non-permeating solutes.
The hydrostatic pressure needed to offset the tendency for water to move from a region of high water potential/concentration to one of low potential/concentration.
Osmotic Pressure can be determined by knowing the Osmotic concentration (or Osmolarity). Osmolarity depends strictly on number of particles per litre of solution, i.e., on [ ] of ALL solute particles in solution. So Osmolarity (in Osmolar units) = Concentration (in Molarity) x Number of particles (Note: salts dissociate in solution and each particle exerts an osmotic pressure).
Iso-osmotic: when two solutions have same number of particles per litre of solution, e.g., 0.1 M CaCl2, 0.15 M NaCl & 0.15 M MgSO4 are all iso-osmotic with Osmolarity = 300 mOsm
Hypo-osmotic: solution with lower number of particles per litre of solution than another,  e.g,  0.1 M NaCl (= 200 mOsm) vs 0.1 M CaCl2 (= 300 mOsm) and 0.15 M MgSO4 (= 300 mOsm)
Hyper-osmotic: solution with higher number of particles per litre of solution than another, e.g, 0.1 M CaCl2 (= 300 mOsm) vs 0.1 M NaCl (= 200 mOsm) and 0.1 M MgSO4 (= 200 mOsm)


A form of nutrition in which soluble compounds are taken up by the organism, either by pinocytosis or by mechanisms capable of transporting one or a few molecules at a time (membrane pumps).

Ossicles or Ossicular chain

In Vertebrates, the three small bones (malleus, incus, and stapes, or hammer, anvil, and stirrup) that are moved by the vibrations of the eardrum (in response to sound generally) and transmit these eardrum vinrations  through the middle ear cavity to the inner ear. In Echinoderms, small calcareous plates forming the skeleton.


A type of arthritis caused by inflammation, breakdown, and eventual loss of cartilage in the joints. Also known as degenerative arthritis.
Among the over 100 different types of arthritis conditions, osteoarthritis is the most common, affecting usually the hands, feet, spine, and large weight-bearing joints, such as the hips and knees. Also called degenerative joint disease.


Refers to any structure that facilitates the formation of bone structure. Commonly used to describe the properties of various types of bone grafts and bone graft substitutes.

Osteogenesis imperfecta (OI)

Genetic disorder that is also characterized by easily fractured bones.


Any substance that stimulates bone formation. Bone morphogenetic proteins are osteoinductive.


The diminishing of bone density, typically related to aging and menopause in women.


Taking out part or all of a bone, or cutting into or through bone.


Surgical construction of an artificial opening (stoma) for external fistulization of a duct or vessel by insertion of a tube with or without a supportive stent.


The process of creating bone, that is of transforming cartilage (or fibrous tissue) into bone.

Otitis media

Excessive fluid, inflammation, and/or infection in the middle ear, caused by an inability to drain out through the eustachian tube.


A condition associated with diseases of the inner ear, characterized by destruction of the capsular bone in the middle ear and growth of a weblike bone that attaches to the stapes.  May result in hearing disorders.


An instrument that allows seeing down the ear canal to see the ear drum and the external ear canal.

Ototoxic drugs

Drugs that can be poisonous to or have a deleterious effect on the
eighth nerve or on the organs of hearing and balance.


the place where a sewer, drain, or stream discharges; the outlet or structure through which reclaimed water or treated effluent is finally discharged to a receiving water body.


A species or group of species that is closely related to the group of species being studied, but clearly not as closely related as any study-group members are to each other. In a cladistic analysis, any taxon used to help resolve the polarity of characters, and which is hypothesized to be less closely related to each of the taxa under consideration than any are to each other.
A taxon that is not part of the ingroup but that is included in a phylogenetic analysis in order to provide information about the root of the ingroup and to help differentiate between apomorphies and plesiomorphies in the ingroup.

Ovarian cycle

The cyclic recurrence of the follicular phase, ovulation, and the luteal phase in the mammalian ovary, regulated by hormones.

Ovarian follicle

A developing oocyte and the specialized cells surrounding it; located near the surface of the ovary; following ovulation, forms the corpus luteum.


A female in which the ovaries have been removed


(1) In flowers, the portion of a carpel in which the egg-containing ovules develop. (2) In animals, the structure that produces female gametes and reproductive hormones


Conventionally defined as a body mass index (weight/height squared) greater than 25.


A tube passing from the ovary to the vagina in invertebrates or to the uterus in vertebrates.


Referring to a type of development in which young hatch from eggs that are retained in the mother's uterus.


The deposition of eggs by the mother into a host or the environment.


A tubular organ that is used for oviposition, i. e., the laying of eggs.
In some insects, especially parasitoid wasps, the ovipositor can be very long.


The release of an egg from ovaries. In humans, an ovarian follicle releases an egg during each menstrual cycle.


A structure that develops in the plant ovary and contains the female gametophyte.

Ovum (Plural = Ova)

The female gamete; the haploid, unfertilized egg, which is usually a relatively large, nonmotile cell.


Containing oxygen.


Chemical reaction involving the addition or combination of oxygen with another material.
The loss of electrons from a substance involved in a redox reaction.
Chemical transformation of a substance by combining it with oxygen or through the loss of hydrogen or electrons. Opposite of reduction.
Oxidized = A substance that has undergone oxidation.


to combine with oxygen.

Oxidative phosphorylation

The production of ATP using energy derived from the redox reactions of an electron transport chain.

Oxidative stress

A state in which the chemical balance is shifted towards an abundance of molecules that ‘oxidise’. That is, they are prone to donate electrons to other atoms and molecules.

Oxidizing agent

The electron acceptor in a redox reaction.

Oxygen concentrator

A device that separates oxygen from room air, supplying up to 95% oxygen at low flow rates.

Oxygen debt

In muscle, the cumulative deficit of oxygen that develops during strenuous exercise when the supply of oxygen is inadequate for the demand.  Then ATP is produced anaerobically by glycolysis, and the resulting pyruvic acid is converted to lactic acid, which is subsequently metabolized when adequate oxygen is available.

Oxygen demand

The need for molecular oxygen to meet the needs of biological and chemical processes in water.Even though very little oxygen will dissolve in water, it is extremely important in biological and chemical processes.


An isotope of oxygen that blocks ultra-violet radiation. Normally found in the stratosphere