1. A general or fundamental doctrine or tenet. SEE ALSO: law, rule, theorem. 2. The essential ingredient in a substance, especially one that gives it its distinctive quality or effect. [L. principium, a beginning, fr. princeps, chief]
- active p. a constituent of a drug, usually an alkaloid or glycoside, upon the presence of which the characteristic therapeutic action of the substance largely depends.
- antianemic p. the material in liver (and certain other tissues) that stimulates hemopoiesis in pernicious anemia; for practical purposes, the antianemic effect of extracts from such tissues is approximately equivalent to the content of vitamin B12.
- Bernoulli p. SYN: Bernoulli law.
- bitter principles a class of plant substances with a bitter taste that produce a reflexive increase in saliva secretion as well as secretion of digestive juices.
- closure p. in psychology, the p. that when one views fragmentary stimuli forming a nearly complete figure ( e.g., an incomplete rectangle) one tends to ignore the missing parts and perceive the figure as whole. See gestalt.
- consistency p. in psychology, the desire of the human being to be consistent, especially in attitudes and beliefs; theories of attitude formation and change based on the consistency p. include balance theory, which suggests that one seeks to avoid incongruity in one's various attitudes. SEE ALSO: cognitive dissonance theory.
- Fick p. SYN: Fick method.
- follicle-stimulating p. SYN: follitropin.
- founder p. the conditional probabilities of the frequencies of a set of genes at any future date depend on the initial composition of the founders of the population and have in general no tendency to revert to the composition of the population from which the founders were themselves derived.
- hematinic p. the p. previously thought to be produced by the action of Castle intrinsic factor upon an extrinsic factor in food, now recognized as vitamin B12.
- Huygens p. used in ultrasound technology; the p. that any wave phenomenon can be analyzed as the sum of many simple sources properly chosen with regard to phase and amplitude.
- p. of inertia SYN: repetition-compulsion p..
- Le Chatelier p. SYN: Le Chatelier law.
- luteinizing p. SYN: lutropin.
- mass action p. the fundamental p. in epidemic theory : the incidence of an infectious disease is determined by the product of the current prevalence and the number of susceptibles in the population. SEE ALSO: serial interval, infection transmission parameter.
- melanophore-expanding p. SYN: melanotropin.
- Mitrofanoff p. use of a catheterizable channel (appendix, bowel, ureter) to drain the bladder as an alternative to the urethra. SEE ALSO: appendicovesicostomy.
- nirvana p. in psychoanalysis, the p. that expresses the tendency to attain a conflict-free state of freedom from pain or worry.
- organic p. SYN: proximate p..
- pain-pleasure p. a psychoanalytic concept that, in human psychic functioning, the person tends to seek pleasure and avoid pain; a term borrowed by experimental psychology to denote the same tendency of an animal in a learning situation. SYN: pleasure p..
- Pauli exclusion p. the theory limiting the number of electrons in the orbit or shell of an atom; that it is not possible for any two electrons to have all four quantum numbers identical.
- pleasure p. SYN: pain-pleasure p..
- proximate p. in chemistry, an organic compound that may exist already formed as a part of some other more complex substance ( e.g., various sugars, starches, and albumins). SYN: organic p..
- reality p. the concept that the pleasure p. in personality development is modified by the demands of external reality; the p. or force that compels the growing child to adapt to the demands of external reality.
- repetition-compulsion p. in psychoanalysis, the impulse to redramatize or reenact earlier emotional experiences or situations. SYN: p. of inertia.
- ultimate p. one of the chemical elements.

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prin·ci·ple 'prin(t)-sə-pəl n
1) a comprehensive and fundamental law, doctrine, or assumption
2) an ingredient (as a chemical) that exhibits or imparts a characteristic quality <the active \principle of a drug>

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prin·ci·ple (prinґsĭ-pəl) [L. principium] 1. a chemical component. 2. a substance on which certain of the properties of a drug depend. 3. a law of conduct.

Medical dictionary. 2011.