Godwin's law, also known as Godwin's rule of Hitler analogies, is a statement maintaining that if any online discussion continues long enough, someone will almost certainly compare someone else to Hitler. Typically, the comment likens someone to Hitler or calls that person a Nazi, and the individual described in that way is often a participant in the discussion. The law is thought to apply to conversations about any conceivable topic.
Mike Godwin, an American lawyer and author, formulated the law in reference to Usenet newsgroup discussions but it is now widely considered applicable to any online channel and particularly to social media sites like Facebook and Twitter, where large groups of loosely-connected individuals frequently engage in extended and contentious arguments about all sorts of things.
Godwin originally developed the idea in 1990 as “a natural law of Usenet," expressed in the following statement: “As an online discussion grows longer, the probability of a comparison involving Hitler approaches 1." (A probability of 1 is a certainty.) A corollary to Godwin's law states that once Hitler is mentioned, that discussion is ended. The implication is that the level of discourse has devolved to the degree that further communication is pointless. According to Usenet tradition, whoever mentioned Hitler is deemed to have lost the argument.
Godwin’s purpose in creating the law was to make people more thoughtful in online communications, as he said, “I wanted folks who glibly compared someone else to Hitler to think a bit harder about the Holocaust.” The exercise was, according to Godwin, an experiment in mimetics. In his discussion of the subject, Godwin is credited as the first person to use the word meme to refer to viral online content.
PBS covered Godwin’s law in Three Laws of the Internet: