Appeal to authority is an argumentation technique, in which one refers to a source that claims to have authority. It is an argumentation fallacy because it assumes that authorities or institutions are right. This, however, does not have to be true by definition.

Source A says that premise 1 is correct
Source A is an authority
Therefore A must be correct

Appeal to authority is also known as 'argument from authority' or 'argumentum ad verecundiam', An example of appeal to authority can be seen here in this advertisement, where physicians are used to sell cigarettes.