antiphrasis

from WordNet (r) 3.0 (2006)

antiphrasis
    n 1: the use of a word in a sense opposite to its normal sense
         (especially in irony)

from The Collaborative International Dictionary of English v.0.48

Antiphrasis \An*tiph"ra*sis\, n. [L., fr. Gr. ?, fr. ? to
   express by antithesis or negation.] (Rhet.)
   The use of words in a sense opposite to their proper meaning;
   as when a court of justice is called a court of vengeance.
   [1913 Webster] Antiphrastic