The ad verecundiam fallacy (appeal to authority) occurs when an arguer relies principally upon the established position, title, credential, or reputation of a source to accept that source's opinion as correct beyond doubt (or beyond reasonable doubt). This overlooks that authorities are not only fallible, but also when they are correct it is not because of their holding authority, but rather because of their knowledge and/or reasoning, which can be presented and explained without reference to or reliance upon their authority.

Here is an example:

Beethoven has to have been the greatest composer, ever. This must be true because Rachmaninoff said, "Beethoven was the greatest composer that ever lived. I place him above Bach, above Mozart, above Schubert, above everyone."

Rachmaninoff's worldwide fame and respect as a composer and pianist give him considerable authority in discussions of classical music, but do not guarantee that his opinion about Beethoven is correct. Treating his opinion as though it must be true just because his voice is authoritative, is a fallacy.