Adapted from Wikipedia:

**Occam's razor**(or Ockham's razor) is a principle that generally recommends selecting the competing hypothesis that makes the fewest new assumptions, when the hypotheses are equal in other respects; for instance, if all the hypotheses can sufficiently explain the observed data.

Also known as the logical principle of parsimony, Occam's Razor is often expressed as "The simplest explanation is the best". This is misleading, however, as many people have a definition of "simple" that does not fit it with what the principle suggests. Another way to state it is "do not multiple entities unnecessarily". In other words, if you have two explanations for a phenomenon, whichever one has the fewest undefined/ad hoc terms should be considered correct. Obviously, this only applies to explanations that actually cover all of the relevant facts of a situation.

This is often invoked in vs. debating when there are two or more competing theories to explain an event. Even if there is no actual proof that one or the other is right, whichever theory is favored by Occam's Razor is more likely to be correct.