An ad hominem argument, also known as argumentum ad hominem (Latin, literally argument against the person), personal attack or you-too argument, involves replying to an argument or assertion by attacking the person presenting the argument or assertion rather than the argument itself. It is a logical fallacy.

A (fallacious) ad hominem argument has the basic form:

1. Contention: Claim X is false.
2. Premise: A makes claim X.
3. Co-premise: There is something objectionable about A.

An ad hominem fallacy consists of asserting that someone's argument is wrong and/or he is wrong to argue at all purely because of something discreditable/not-authoritative about the person or those persons cited by him rather than addressing the soundness of the argument itself. The implication is that the person's argument and/or ability to argue correctly lacks authority. Merely insulting another person in the middle of otherwise rational discourse does not necessarily constitute an ad hominem fallacy. It must be clear that the purpose of the characterization is to discredit the person offering the argument, and, specifically, to invite others to discount his arguments. In the past,the term ad hominem was sometimes used more literally, to describe an argument that was based on an individual, or to describe any personal attack. However, this is not how the meaning of the term is typically introduced in modern logic and rhetoric textbooks, and logicians and rhetoricians are in agreement that this use is incorrect.