Argument from authority or appeal to authority is a logical fallacy, where it is argued that a statement is correct because the statement is made by a person or source that is commonly regarded as authoritative. The most general structure of this argument is:
Source A says that p.
Source A is authoritative.
Therefore, p is true.
On the other hand, arguments from authority are an important part of informal logic. Since we cannot have expert knowledge of many subjects, we often rely on the judgments of those who do. There is no fallacy involved in simply arguing that the assertion made by an authority is true. The fallacy only arises when it is claimed or implied that the authority is infallible in principle and can hence be exempted from criticism.
– Argument from authority. In Wikipedia, The Free Encyclopedia. Retrieved 7 December 2009
Boldface and italics by Wikipedia editors, underline by me.
The argument from ignorance, also known as argumentum ad ignorantiam (“appeal to ignorance”), argument by lack of imagination, or negative evidence, is a logical fallacy in which it is claimed that a premise is true only because it has not been proven false, or is false only because it has not been proven true.
– Argument from ignorance. In Wikipedia, The Free Encyclopedia. Retrieved October 18, 2009