n. A figure of speech in which the speaker emphasizes the magnitude of a statement by denying its opposite; a figure of speech in which understatement is used with negation to express a positive attribute; a form of irony
the GNU version of the Collaborative International Dictionary of English
n. A diminution or softening of statement for the sake of avoiding censure or increasing the effect by contrast with the moderation shown in the form of expression; a form of understatement.
The Century Dictionary and Cyclopedia
n. In rhetoric, a figure in which an affirmative is expressed by the negative of the contrary. Thus, “a citizen of no mean city” means one “of an illustrious city.”
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary)
Word Usage
"At first called litotes or meiosis, such understatement came to be called irony, at least by the end of the sixteenth century."