An ad hominem argument, also known as argumentum ad hominem (Latin: "argument to the person", "argument against the man") consists of replying to an argument or factual claim by attacking or appealing to the person making the argument or claim, rather than by addressing the substance of the argument or producing evidence against the claim. It is most commonly used to refer specifically to the ad hominem abusive, or argumentum ad personam, which consists of criticizing or personally attacking an argument's proponent in an attempt to discredit that argument.
Other common subtypes of the ad hominem include the ad hominem circumstantial, or ad hominem circumstantiae, an attack which is directed at the circumstances or situation of the arguer; and the ad hominem tu quoque, which objects to an argument by characterizing the arguer as being guilty of the same thing that he is arguing against.
Ad hominem arguments are always invalid in syllogistic logic, since the truth value of premises is taken as given, and the validity of a logical inference is independent of the person making the inference. However, ad hominem arguments are rarely presented as formal syllogisms, and their assessment lies in the domain of informal logic and the theory of evidence.
On the other hand, the theory of evidence depends to a large degree on assessments of the credibility of witnesses, including eyewitness evidence and expert witness evidence. Evidence that a purported eyewitness is unreliable, or has a motive for lying, or that a purported expert witness lacks the claimed expertise can play a major role in making judgements from evidence.
Argument ad hominem is the converse of appeal to authority, in which the arguer bases the truth value of an assertion on the authority, knowledge or position of the person asserting it. Hence, while an ad hominem argument may make an assertion less compelling, by showing that the person making the assertion does not have the authority, knowledge or position they claim, or has made mistaken assertions on similar topics in the past, it cannot provide an infallible counterargument.