An ad hominem argument is an argument that attacks a claim on the basis of features of the person who holds it. Two different sorts of argument are called "ad hominem arguments." One of these is a fallacious sort of argument; the other is perfectly respectable.

The fallacious version is where you criticize someone's views because of logically irrelevant personal defects. For instance:

His views about relationships must be false because he's a philanderer.

or:

His views about politics must be false because he doesn't know what

he's talking about.

You should remember that authorities no matter how eminent can be wrong, and that scoundrels and fools--even if they are unjustified in their beliefs--might nonetheless turn out to be right. The source of a belief is one thing, and whether there are any good reasons to hold the view is something else.

The respectable argument called an "ad hominem argument" consists in objecting to someone's claim on the grounds that it's incompatible with other views he holds--regardless of whether you regard those other views as correct.

For instance, suppose Max says:

The U.S. Postal Service is very unreliable. I think we should allow private, for-profit companies like FedEx and UPS to compete on an equal footing with the Postal Service.

Then Sally objects:

But Max, you are a communist!

Sally is not just calling Max a name. Sally's point is that Max's previous commitments force him to support state control and oppose private enterprise, and these commitments conflict with the view he's advocating now. This is a perfectly legitimate criticism of Max.

Philosophers generally use the phrase "ad hominem argument" in the second sense.