A logical fallacy in which a person presenting a position on a subject mentions an authority who also holds that position, but may not be an authority in that area. For instance, the statement "Arthur C. Clarke recently released a report showing it necessary to floss three times daily" would be unlikely to impress many people, as Arthur C. Clarke is likely not an expert on dental hygeine. Much of advertising relies on this logical fallacy, as well as on the /Bandwagon fallacy.
Citing a person who is a recognized authority in the field is likely to carry more weight, and deservedly so. But this is still strictly speaking a logical fallacy, especially when the question itself is a matter of opinion or a question of some controversy. While experts are less likely to be mistaken than others, it is not impossible or even uncommon.