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Ad hominem or name-calling? Why the difference matters
"That's an ad hominem!" is one of the most common replies in online arguments, and one of the most often misused. Many comments called ad hominem are rude, but rudeness alone is not the fallacy.
Two different moves
Name-calling attaches a label to a person or group: "idiot", "shill", "traitor". It is a technique of persuasion. It makes the target look bad and invites the audience to share the feeling.
Ad hominem is a fallacy of reasoning. It treats a fact about the person as a reason to reject what they said: "You only support this tax because you work for the government, so your argument is worthless." The attack replaces the answer.
A quick test
Remove the insult and see whether an argument is left. "Your plan ignores rural voters, you clown" still contains a reason (the plan ignores rural voters) that can be checked. The insult is ugly, but the reasoning does not rely on it. "You're a clown, so nobody should listen to your plan" contains nothing else. The insult is doing all the work, and that is the fallacy.
The test also works the other way. Some ad hominem arguments are perfectly polite: "With respect, someone who has never run a business cannot judge this regulation." No insult at all, yet the claim is dismissed because of who made it. That variant is close to the genetic fallacy.
Why it matters
When we reread comments that a public research dataset had labelled ad hominem, most turned out to be plain insults rather than attempts to dismiss an argument because of who made it. That distinction changes what a good reply looks like. To name-calling, the useful answer is often to ignore the label and restate your point. To an ad hominem, the useful answer is to say plainly that who makes an argument does not decide whether it is sound, and to bring the discussion back to the reasons.
It also matters when you check your own writing. An insult in a strong argument weakens it in readers' eyes but does not make it invalid. An ad hominem makes the argument itself fail.
Related moves
Pointing out that a critic does the same thing they criticise is tu quoque, a close cousin of ad hominem. Linking someone to a disliked group to discredit them is guilt by association, and mocking a position instead of answering it is appeal to ridicule.