What is Ad Hominem?

Legal Definition
Ad hominem (Latin for "to the man" or "to the person"), short for argumentum ad hominem, is a logical fallacy in which an argument is rebutted by attacking the character, motive, or other attribute of the person making the argument, or persons associated with the argument, rather than attacking the substance of the argument itself.

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

However, in some cases, ad hominen attacks can be non-fallacious; i.e., if the attack on the character of the person is directly tackling the argument itself. For example, if the truth of the argument relies on the truthfulness of the person making the argument—rather than known facts—then pointing out that the person has previously lied is not a fallacious argument.
-- Wikipedia
Legal Definition
To the person. A term used in logic with reference to a personal argument.
-- Black's Law Dictionary
Legal Definition
To the man; personal.
-- Ballentine's Law Dictionary