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]