Appeal to Authority

You appeal to authority if you back up your reasoning by saying that it is supported by what some authority says on the subject. Most reasoning of this kind is not fallacious. However, it is fallacious whenever the authority appealed to is not really an authority in this subject, when the authority cannot be trusted to tell the truth, when authorities disagree on this subject (except for the occasional lone wolf), when the reasoner misquotes the authority, and so forth. Although spotting a fallacious appeal to authority often requires some background knowledge about the subject or the authority, in brief it can be said that it is fallacious to accept the word of a supposed authority when we should be suspicious.

Example:

You can believe the moon is covered with dust because the president of our neighborhood association said so, and he should know.

This is a fallacious appeal to authority because, although the president is an authority on many neighborhood matters, he is no authority on the composition of the moon. It would be better to appeal to some astronomer or geologist. If you place too much trust in expert opinion and overlook any possibility that experts talking in their own field of expertise make mistakes, too, then you also commit the fallacy of appeal to authority.
Example:

Of course she's guilty of the crime. The police arrested her, didn't they? And they're experts when it comes to crime.