The Latin phrase ad hominem means "to the person" and is shorthand for argumentum ad hominem or "an argument against the person." It refers to a logical fallacy used during debate that relies on attacking the other person rather than arguing the substance of the case being made.

For instance, if you were attempting to convince someone that the earth was round and not flat, and made multiple arguments including the use of satellite photos of the earth from space, an ad hominem response would be for the other person to say, "Yeah, but you're a socialist, so I don't believe you." The response is an attack on the person (whether true or not), and not on the merits of the argument.