noun.
A fallacious or illogical argument or conclusion.
The Century Dictionary and Cyclopedia
noun.
In logic, fallacious argument or false reasoning; reasoning which is false in form—that is, in which the conclusion does not follow from the premises; a conclusion unwarranted by the premises.
the GNU version of the Collaborative International Dictionary of English
noun.
A reasoning which is false in point of form, that is, which is contrary to logical rules or formulæ; a formal fallacy, or pseudo-syllogism, in which the conclusion does not follow from the premises.