Denial of the contrary; understatement that intensifies, as when a politician once said to his garrulous subordinate, "A period of silence from you would now be not unwelcome." In litotes, "more is understood than is said," as in "He is not the wisest man in the world" when we mean "He is a fool" (Peacham). Sister Miriam Joseph remarks: "Litotes is related to what the logicians call equipollence or obversion, which consists in expressing a thought by denying its contradictory."
A Handlist of Rhetorical Terms, Second Edition, Richard A. Lanham, 1991