To clarify, an appeal to authority is a logical fallacy when it is argued that a statement is correct because the source is regarded as authoritative. Citing the credentials of one's source can help us weigh our decision to accept the assertion in the absence of further evidence, but ultimately, either an assertion is correct or it is not.
Here is the general structure of a fallacious appeal to authority:
1. Source A says that x is true.
2. Source A is authoritative.
3. Therefore, x is true.
With that said, arguments from authority are essential in situations of informal logic. Most people lack expertise in numerous fields, so we tend to rely on the judgments of experts in those fields. There is no fallacy in the proposition that an assertion made by an authority is true. The fallacy comes about when one claims, or implies, that the cited authority is incapable of error in the subject under review, and is therefore immune to criticism.