Relying on experts is not a fallacy; nobody can check everything themselves. The appeal to authority becomes fallacious when the authority's word is used in a way that does not actually support the claim. Three patterns cover most cases. The authority is outside their field: a famous physicist's opinion on nutrition carries no special weight, nor does an actor's on vaccines. The authority is a lone voice against the consensus of their own field, presented as if they spoke for it. Or the authority is vague and uncheckable: 'scientists say', 'experts agree', 'doctors recommend' with no one named. A reasonable appeal names a relevant expert, reflects what most experts in that field hold, and concerns a question experts can actually settle. Notice that the fallacy is about the reasoning, not the conclusion: the celebrity might be right, but their fame gives you no reason to think so. After this Concept you can decide whether an appeal to authority in a claim is a fair reliance on relevant expertise or a fallacy, and say which of the three patterns it falls into.
The appeal to authority fallacy is relying on an expert's word in an improper way, often to brush aside real evidence. In critical thinking, expert testimony counts as legitimate but indirect evidenc…