An informal fallacy is one that cannot be detected merely by examining the form of the argument but must be detected in some other way. It is any other argument that does not properly establish the supported conclusion. An argument contains an informal fallacy when at least one of its premises is not true, or when the rules of inference are not properly respected.

Ruggero J. Aldisert, Agatha D. Aldisert, Logic for Lawyers- A Guide …, loc. 774