litotes

A figure of speech consisting of an understatement in which an affirmative is expressed by negating its opposite, as in This is no small problem.

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.”

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.

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

understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary)