[L. principium, foundation]

1. A constituent of a compound representing its essential properties.
2. A fundamental truth.
3. An established rule of action.

active principle

The ingredient of a pharmaceutical preparation that produces the therapeutic action.

air gap principle

A procedure to decrease the amount of scattered radiation reaching the radiographic film by increasing the object-image receptor distance.

antidiuretic principle

A term formerly used to indicate “antidiuretic hormone.”
SEE: antidiuretic hormone

Arndt-Schultz principle

SEE: Arndt-Schultz principle

Fick principle

SEE: Fick, Adolf Eugen

gastrointestinal principle

An archaic term used for hormones, such as cholecystokinin, gastrin, and secretin, which are secreted by mucosal cells of the gastrointestinal tract and absorbed into the blood.

oxytocic principle

An obsolete term for oxytoxin.

labeled line principle

A hypothesis to explain how different nerves, all of which use the same physiological principles in transmitting impulses along their axons, are able to generate different sensations. Structurally similar nerves can generate distinct sensory perceptions if they are connected to unique neurons in the central nervous system that are capable of decoding similar nerve signals in different ways.

pleasure principle

In psychoanalytic theory, the idea that people strive to avoid pain, hunger, and physical or psychological stresses in favor of pleasant experiences, e.g., food, sex, and narcissistic satisfaction

precautionary principle

A risk management principle, originally developed in the environmental movement, based on the concept of avoiding any new action, e.g., introducing a new technology or a new drug, that carries a hypothetical risk for human health or the environment, regardless of whether the hypothesis has been subjected to formal testing.

proximate principle

An obsolete term for a substance that may be extracted from its complex form without destroying or altering its chemical properties.

reality principle

In psychoanalysis, the idea that the striving for narcissistic pleasure can never be absolute but must be balanced against competing demands placed on the self by other persons and situations.