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.

(fluid dynamics) A law that states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces.

(geometry) A development of the method of indivisibles, dealing with two-dimensional area or three-dimensional volume based on the intersections of lines or planes surrounding a region. It was an early step toward integral calculus.

plural of reciprocity principle

In the neo-Gricean semantics and pragmatics of Laurence R. Horn: a reformulation of Paul Grice's maxim of relation combined with the second submaxim of quantity and the third and fourth submaxims of manner, stating: "Say no more than you must (given Q)", and implying that there is no reason to make a stronger statement if the extra information can be contributed by implicature.

In the neo-Gricean semantics and pragmatics of Laurence R. Horn: a reformulation of Paul Grice's maxim of quantity combined with the first two submaxims of manner, stating: "Say as much as you can (given R)", and leading to the implicature that if the speaker did not make a stronger statement (or say more), then its denial is (implied to be) true.

A principle stating that the most effective design for a product is one that is "Most Advanced, Yet Acceptable", or one that combines familiarity and novelty.

(physics) the principle that the laws of gravity are independent of velocity and position

(physics, quantum mechanics) The principle that there is an absolute limit on the combined accuracy of certain pairs of simultaneous, related measurements, especially that of the position and momentum of a particle. Originally posited as a problem of measurement, it was soon refined as an inherent property of the universe.

(chemistry) A principle stating that, if a chemical system at equilibrium experiences a change in concentration, temperature, or total pressure, the equilibrium will shift in order to minimize that change.