Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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