The Collaborative International Dictionary of English v.0.48:

Sophism \Soph"ism\, n. [F. sophisme, L. sophisma, fr. Gr. ?, fr.
   ? to make wise, ? to be become wise, to play the sophist, fr.
   ? wise.]
   The doctrine or mode of reasoning practiced by a sophist;
   hence, any fallacy designed to deceive.
   [1913 Webster]

         When a false argument puts on the appearance of a true
         one, then it is properly called a sophism, or
         "fallacy".                               --I. Watts.
   [1913 Webster]

         Let us first rid ourselves of sophisms, those of
         depraved men, and those of heartless philosophers. --I.
                                                  Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sophism
    n 1: a deliberately invalid argument displaying ingenuity in
         reasoning in the hope of deceiving someone [syn: sophism,
         sophistry, sophistication]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "sophism":
   apparent soundness, argument, argument by analogy,
   argumentum ad baculum, argumentum ad captandum,
   argumentum ad hominem, bad case, begging the question, casuistry,
   circular argument, circularity, claptrap, crowd-pleasing argument,
   deception, deceptiveness, delusion, disingenuousness, distortion,
   empty words, equivocalness, equivocation, evasive reasoning,
   fallaciousness, fallacy, formal fallacy, hysteron proteron,
   illogicality, insincere argument, insincerity, invalidity,
   irrationality, jesuitism, jesuitry, logical fallacy,
   material fallacy, mere rhetoric, misapplication, moonshine,
   mystification, non sequitur, obfuscation, obscurantism,
   oversubtlety, paralogism, perversion, petitio principii,
   philosophism, plausibility, plausibleness, pseudosyllogism,
   rationalization, solecism, sophistical reasoning, sophistication,
   sophistry, special pleading, speciosity, specious reasoning,
   speciousness, spuriousness, subtlety, unsoundness, verbal fallacy,
   vicious circle, vicious reasoning, weak point