An ad hominem (Latin for “to the man” or “to the person”), short for argumentum ad hominem, means responding to arguments by attacking a person’s character, rather than addressing the content of their arguments. When used inappropriately, it is a fallacy in which a claim or argument is dismissed on the basis of some irrelevant fact or supposition about the author or the person being criticized. Ad hominem reasoning is not always fallacious, for example, when it relates to the credibility of statements of fact or when used in certain kinds of moral and practical reasoning.

Fallacious ad hominem reasoning is normally categorized as an informal fallacy, more precisely as a genetic fallacy, a subcategory of fallacies of irrelevance.