The link between ray optics and wave optics, stating (in its original strong form) that the path taken by a ray between two given points is the path that can be travelled in the least time, or (in a weaker form that is true for all cases) in a time that is "stationary" with respect to variations of the path — so that a deviation in the path causes, at most, a second-order change in the traversal time.

Any scientific principle that expresses various reciprocal relationships for the behaviour of a physical system.

(astronomy) The assumption that the universe, on a large enough scale, is homogeneous and isotropic.

plural of dormitive principle

(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.

(relativity) The principle that only the motion of objects relative to one another can be measured, and that there is no absolute frame of reference.

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

(quantum mechanics) The Heisenberg uncertainty principle.