Source : Webster's Revised Unabridged Dictionary (1913)

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.