Definition
Ad hominem is Latin for "to the person." It names a fallacy where a debater responds to an argument by attacking the character, motives, appearance, or affiliations of the person who made it, rather than addressing the argument's actual claim, evidence, or reasoning. Even if the personal attack is accurate, it does nothing to show that the argument itself is wrong.
Example
An opponent argues that a proposed tax increase would fund useful public services. A debater responds: "Of course you'd say that, you work for the government." That sentence says nothing about whether the tax increase actually funds useful services. It dismisses the speaker instead of engaging the claim.
Common mistakes
Debaters often confuse ad hominem with a legitimate critique of source credibility. Pointing out that a cited study was funded by a company with a direct financial stake in its outcome is not ad hominem: it is a real argument about evidence quality, made about the source, not the person making the claim in the room. The fallacy is specifically attacking the person in front of you as a substitute for engaging their argument, not any mention of who produced a piece of evidence.