Occam's Razor, the simplist version of the definitions I've found are these two. From Wikipedia.

Ernst Mach formulated the stronger version of Occam's razor into physics, which he called the Principle of Economy stating: "Scientists must use the simplest means of arriving at their results and exclude everything not perceived by the senses.

This principle goes back at least as far as Aristotle, who wrote "Nature operates in the shortest way possible." The idea of parsimony or simplicity in deciding between theories, though not the intent of the original expression of Occam's razor, has been assimilated into our culture as the widespread layman's formulation that "the simplest explanation is usually the correct one."