Wordnet 3.0
NOUN (1)
1.
the use of a word in a sense opposite to its normal sense (especially in irony);
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
WordNet (r) 3.0 (2006):
antiphrasis
n 1: the use of a word in a sense opposite to its normal sense
(especially in irony)