An appeal to authority is an informal logical fallacy. It can be defined as the attempt to justify one's argument by referring to an "authoritative source" who holds the same belief as the arguer. These sources often include (but are not limited to) political figures, scientists, philosophers, and texts. It is rarely seen in academic debate, as editors and peer reviewers of journal articles routinely vet for such errors. However, in discussions of public policy issues as well as everyday human discourse, the appeal to authority is one of the most common rhetorical tricks.
More specifically, it refers to citing a recognized authority for something in which they have no special expertise, using the fact that they are knowledgeable about one field to create the impression they are knowledgeable about all fields. For instance, citing Robert Bork on Constitutional Law is perfectly reasonable. However, citing Robert Bork on the best way to repair a broken automobile is a logical fallacy, because Bork presumably knows no more about car engines than the average American.
Liberals regularly use this fallacy by, for instance, citing Albert Gore, who has no scientific training, as an expert on global warming. More generally, liberals often assume that any book espousing a liberal position is automatically authoritative and unquestionable.