Fallacy

A mistake or defect in reasoning  - especially of the sort likely to go undetected and likely to possess a "veneer" of considerable persuasiveness - an argument involving improper reasoning; also the conclusion reached by such reasoning. Either:

• when it is possible for the premises - reasons given as providing support for a conclusion - to be true, yet the conclusion false;

Although usually leading to false, erroneous, or suspect conclusions, sound conclusions may sometimes be reached despite the faulty means.

(see also: argument, reasoning, Van Gogh fallacy, thinking error)