(idiomatic, law) A legally enforceable, but incompletely specified, agreement between parties that identifies the fundamental terms that are intended to be or are agreed upon.

(physics) The assumption that each point on a wavefront may be regarded as a source of secondary waves and that the position of the wavefront at a later time is determined by the envelope of these secondary waves; proposed in order to explain the fact that light travels in a straight line and casts sharp shadows.

(idiomatic, rhetoric, logic, linguistics) A type of tautology in which an item is explained in terms of the item itself, only put in different (usually more abstract) words.

The empirical principle that most systems will work best if they are kept simple rather than made complex.

(fluid dynamics) The principle which states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy.

(physics) The principle that the inertia of a body arises from its relation to the totality of all other bodies in the universe.

(physics) any of several principles, in relativity, concerned with the uniformity of physical measurements in different frames of reference

(quantum mechanics) The Heisenberg uncertainty principle.

(quantum mechanics) The Heisenberg uncertainty principle.

a peculiar principle once erroneously supposed to form the basis of all vegetable extracts