fallacy

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A false notion.
2. n. A statement or an argument based on a false or invalid inference.
3. n. Incorrectness of reasoning or belief; erroneousness.
4. n. The quality of being deceptive.

Century Dictionary and Cyclopedia

1. n. Deceptiveness; deception; deceit; deceitfulness; that which is erroneous, false, or deceptive; that which misleads; mistake.
2. n. Specifically— A false syllogism; an invalid argumentation; a proposed reasoning which, professing to deduce a necessary conclusion, reaches one which may be false though the premises are true, or which, professing to be probable, infers something that is really not probable, or wants the kind of probability assigned to it. A fallacy is either a sophism or a paralogism, according as the deceit is intentional or not. But the word paralogism is also used to signify a purely logical fallacy—that is, a formal fallacy, or a direct violation of the canons of syllogism. Logicians enumerate as many different kinds of formal fallacy as they give of canons of syllogism, from four to eight. See below.
3. n. The fallacy of accident, arising when a syllogism is made to conclude that, because a given predicate may be truly affirmed of a given subject, the same predicate may be truly affirmed respecting all the accidents of that subject.
4. n. The fallacy of speech respective and speech absolute, occurring when a proposition is affirmed with a qualification or limitation in the premises, but virtually without the qualification in the conclusion.
5. n. The fallacy of irrelevant conclusion, or ignoration of the elench, occurring when the disputant, professing to contradict the thesis, advances another proposition which contradicts it in appearance but not in reality.
6. n. The fallacy of the consequent, or non sequitur, an argument from consequent to antecedent, which may really be a good probable argument.
7. n. Begging the question, or the petitio principii, a syllogism, valid in itself, but in which that is affirmed as a premise which no man who doubts the conclusion would admit.
8. n. The fallacy of false cause, arising when, in making a reductio ad absurdum, besides the proposition to be refuted, some other false premise is introduced.
9. n. The fallacy of many interrogations in which two or more questions are so proposed that they appear to be but one: as, “Have you lost your horns?” a question which implies that you had horns.

Wiktionary

1. n. Deceptive or false appearance; deceitfulness; that which misleads the eye or the mind; deception.
2. n. logic An argument, or apparent argument, which professes to be decisive of the matter at issue, while in reality it is not. A specious argument.

GNU Webster's 1913

1. n. Deceptive or false appearance; deceitfulness; that which misleads the eye or the mind; deception.
2. n. (Logic) An argument, or apparent argument, which professes to be decisive of the matter at issue, while in reality it is not; a sophism.

WordNet 3.0

1. n. a misconception resulting from incorrect reasoning