Definition
An appeal to authority argues that a claim must be true because a credentialed or respected figure said so, without presenting the underlying evidence or reasoning that would let a judge evaluate the claim independently. Expert opinion can be legitimate evidence, but it becomes a fallacy when it is offered as a substitute for reasoning rather than as one input to it.
Example
"A famous economist said this policy will work, so it will work." That sentence tells the judge nothing about why the policy works. It also glosses over whether the economist has relevant expertise in this specific area, whether other economists disagree, and what the actual mechanism is.
Common mistakes
Not every citation of an expert is fallacious. Citing a specialist's finding, and then explaining the reasoning or data behind it, is normal and often necessary in a debate round. The fallacy specifically involves treating the authority's identity as the argument itself, rather than as a pointer to evidence you then explain. A good test: if you removed the expert's name and title, would anything of substance remain in the sentence? If not, it is an appeal to authority rather than a supported claim.