Fallacy \Fal"la*cy\, n.; pl. Fallacies. [OE. fallace, fallas, deception, F. fallace, fr. L. fallacia, fr. fallax deceitful, deceptive, fr. fallere to deceive. See Fail.]

1. Deceptive or false appearance; deceitfulness; that which misleads the eye or the mind; deception.

Winning by conquest what the first man lost, By fallacy surprised. --Milton.

2. (Logic) An argument, or apparent argument, which professes to be decisive of the matter at issue, while in reality it is not; a sophism.

Syn: Deception; deceit; mistake.

Usage: Fallacy, Sophistry. A fallacy is an argument which professes to be decisive, but in reality is not; sophistry is also false reasoning, but of so specious and subtle a kind as to render it difficult to expose its fallacy. Many fallacies are obvious, but the evil of sophistry lies in its consummate art. ``Men are apt to suffer their minds to be misled by fallacies which gratify their passions. Many persons have obscured and confounded the nature of things by their wretched sophistry; though an act be never so sinful, they will strip it of its guilt.'' --South.

Source: Webster's Revised Unabridged Dictionary (1913)