[n] understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary); "saying `I was not a little upset' when you mean `I was very upset' is an example of litotes"
In rhetoric, litotes is a figure of speech in which the speaker either strengthens or weakens the emphasis of a claim by denying its opposite. The literal meaning of a litotes is "not X (but not necessarily Y)", and a litotes can be used as an understatement, actually meaning "very much Y," or to express ambivalence. Like many figures of speech, litotes is better understood in some cultural and linguistic contexts than in others.
rhetorical device, understatement