(mathematics) The theorem which states that any partition of a finite set of n elements into m (< n) subsets (allowing empty subsets) must include a subset with two or more elements; any of certain reformulations concerning the partition of infinite sets where the cardinality of the unpartitioned set exceeds that of the partition (so there is no one-to-one correspondence).

(physics) The observation that, out of the great range of all possible conditions on Earth, the actual conditions are all “just right” in order to support life.

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

(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.

(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.

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

(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.

(quantum mechanics) The Heisenberg uncertainty principle.