A formal fallacy is any violation of any of the six rules of the categorical syllogism or the rules of the hypothetical or disjunctive-alternative syllogism. It is an argument whose conclusion could be false even if all its premises are true. It can be detected merely by examining the form (hence its name) or structure of the argument.

Ruggero J. Aldisert, Agatha D. Aldisert, Logic for Lawyers- A Guide …, loc. 773