The Collaborative International Dictionary of English v.0.48:

Fallacy \Fal"la*cy\ (f[a^]l"l[.a]*s[y^]), n.; pl. Fallacies
   (f[a^]l"l[.a]*s[i^]z). [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.
      [1913 Webster]

            Winning by conquest what the first man lost,
            By fallacy surprised.                 --Milton.
      [1913 Webster]

   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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

fallacy
    n 1: a misconception resulting from incorrect reasoning [syn:
         fallacy, false belief]

Moby Thesaurus II by Grady Ward, 1.0:

165 Moby Thesaurus words for "fallacy":
   Albigensianism, Arianism, Catharism, Ebionitism, Erastianism,
   Gnosticism, Jovinianism, Lollardy, Manichaeanism, Manichaeism,
   Monophysism, Monophysitism, Pelagianism, Waldensianism, Wyclifism,
   aberrancy, aberration, antinomianism, apparent soundness, argument,
   argument by analogy, argumentum ad baculum,
   argumentum ad captandum, argumentum ad hominem, bad case,
   bamboozlement, befooling, begging the question, bluffing,
   calculated deception, casuistry, circular argument, circularity,
   circumvention, claptrap, conning, crowd-pleasing argument,
   deceiving, deception, deceptiveness, defectiveness, defrauding,
   delusion, delusiveness, deviancy, disingenuousness, distortion,
   dupery, elusion, emanatism, empty words, enmeshment, ensnarement,
   entanglement, entrapment, equivocalness, equivocation, errancy,
   erroneousness, error, evasion, evasive reasoning, fallaciousness,
   false doctrine, falsehood, falseness, falsity, fault, faultiness,
   flaw, flawedness, flimflam, flimflammery, fond illusion, fooling,
   formal fallacy, hallucination, hamartia, heresy, heterodoxy,
   hoodwinking, hylotheism, hysteron proteron, illusion,
   inconsistency, insincere argument, insincerity, inveracity,
   jesuitism, jesuitry, kidding, logical fallacy, material fallacy,
   mere rhetoric, mirage, misapplication, misbelief, miscalculation,
   misconception, misconstruction, misdoing, misfeasance,
   misinterpretation, misjudgment, mistake, misunderstanding,
   moonshine, mystification, non sequitur, obfuscation, obscurantism,
   outwitting, overreaching, oversubtlety, pantheism, paralogism,
   peccancy, perversion, petitio principii, phantasm, philosophism,
   plausibility, plausibleness, pseudosyllogism, putting on, quibble,
   rationalization, self-contradiction, self-deception, sin,
   sinfulness, snow job, solecism, song and dance, sophism,
   sophistical reasoning, sophistication, sophistry, special pleading,
   speciosity, specious reasoning, speciousness, spoofery, spoofing,
   spuriousness, subterfuge, subtlety, swindling, trickiness,
   tricking, truthlessness, unorthodoxy, untrueness, untruth,
   untruthfulness, verbal fallacy, vicious circle, vicious reasoning,
   victimization, vision, weak point, willful misconception,
   wishful thinking, wrong, wrongness