"Godwin's Law" is Defined by Mike Godwin
Godwin's Law (also known as Godwin's Rule of Nazi Analogies) is a humorous observation coined by Mike Godwin in 1990, and which has become an Internet adage.
It states: "As a Usenet discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches 1."
Godwin's Law is often cited in online discussions as a deterrent against the use of arguments in the widespread reductio ad Hitlerum form. The rule does not make any statement about whether any particular reference or comparison to Adolf Hitler or the Nazis might be appropriate, but only asserts that the likelihood of such a reference or comparison arising increases as the discussion progresses. It is precisely because such a comparison or reference may sometimes be appropriate, Godwin has argued, that overuse of Nazi and Hitler comparisons should be avoided, because it robs the valid comparisons of their impact.
Although in one of its early forms Godwin's Law referred specifically to Usenet newsgroup discussions, the law is now applied to any threaded online discussion: electronic mailing lists, message boards, chat rooms, and more recently blog comment threads and wiki talk pages.