A pattern of reasoning that feels persuasive but is logically broken — the premises don't actually support the conclusion, even when they happen to be true.

An argument is one or more premises offered to support a conclusion. A fallacy is an error in the reasoning that links them — the premises are presented as supporting the conclusion, but they don't provide the needed support. That is different from a factual error, which is just being wrong about the world ("Columbus is the capital of the USA"). A fallacy can have entirely true premises and still be a fallacy, because the connection is what fails.

• Factual error: the facts are wrong.
• Fallacy: the facts may be fine, but they don't justify the conclusion drawn from them.

Crucially, a fallacious argument can still reach a true conclusion — by luck, not by logic. So spotting a fallacy tells you the argument gives you no good reason to accept the conclusion; it does not tell you the conclusion is false.

Tip: Two-part test — "Are the premises actually relevant to the conclusion, and do they actually establish it?" If either answer is no, you may be looking at a fallacy.