Occam's razor (or Ockham's razor) is often expressed in Latin as the lex parsimoniae (translating to the law of parsimony, law of economy or law of succinctness) is a heuristic that states "entities must not be multiplied beyond necessity" (entia non sunt multiplicanda praeter necessitatem). This may be alternatively phrased as pluralitas non est ponenda sine necessitate ("plurality should not be posited without necessity").  Occam's Razor is attributed to the 14th-century English logician, theologian and Franciscan friar William of Ockham. When competing hypotheses are equal in other respects, the principle recommends selection of the hypothesis that introduces the fewest assumptions and postulates the fewest entities while still sufficiently answering the question. It is in this sense that Occam's razor is usually understood." It serves an important part of science and abductive reasoning.

The principle is popularly summarized, but misleadingly, as "the simplest explanation is usually the correct one." Simplest referring to the theory with the fewest new assumptions and not by the time or number of words it takes to express the theory.

To quote Isaac Newton, "We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances. Therefore, to the same natural effects we must, so far as possible, assign the same causes."

In science, Occam’s razor is used as a heuristic (rule of thumb) to guide scientists in the development of theoretical models rather than as an arbiter between published models. In the scientific method, Occam's razor is not considered an irrefutable principle of logic, and certainly not a scientific result.

In 2005 Marcus Hutter mathematically proved that shorter computable theories have more weight when calculating the expected value of an action across all computable theories which perfectly describe previous observations.