The use of a word in a sense opposite to its normal sense (especially in irony)

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
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.

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1530s, from Latin antiphrasis , from Greek antiphrasis , from antiphrazein "to express (something) by the opposite," from anti- (see anti- ) + phrazein "to consider, to express" (see phrase (n.)).

Wiktionary Word definitions in Wiktionary
n. (context rhetoric English) Use of a word or phrase in a sense not in accord with its literal meaning, especially for ironic or humorous effect

WordNet Word definitions in WordNet
n. the use of a word in a sense opposite to its normal sense (especially in irony)