an·tiph·ra·sis 
(ăn-tĭfrə-sĭs)
(ăn-tĭfrə-sĭs)n.
The use of a word or phrase in a sense contrary to its normal meaning for ironic or humorous effect, as in a mere babe of 40 years.
[Late Latin, from Greek, from antiphrazein, to express by the opposite : anti-, anti- + phrazein, to speak; see PHRASE.]