An appeal to authority is an argument from the fact that a person judged to be an authority affirms a proposition to the claim that the proposition is true.
Appeals to authority are always deductively fallacious; even a legitimate authority speaking on his area of expertise may affirm a falsehood, so no testimony of any authority is guaranteed to be true.
However, the informal fallacy occurs only when the authority cited either (a) is not an authority, or (b) is not an authority on the subject on which he is being cited. If someone either isn’t an authority at all, or isn’t an authority on the subject about which they’re speaking, then that undermines the value of their testimony.
Example
(1) Marilyn vos Savant says that no philosopher has ever successfully resolved the problem of evil.
Therefore:
(2) No philosopher has ever successfully resolved the problem of evil.
This argument is fallacious because Marilyn vos Savant, though arguably an authority, is not an authority on the philosophy of religion. Her judgement that no philosopher has ever successfully resolved the problem of evil therefore carries little evidential weight; if there were a philosopher somewhere that had successfully resolved the problem then there’s a good chance that Marilyn vos Savant wouldn’t know about it. Her testimony is therefore insufficient to establish the conclusion of the argument.