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.
source: WordNet 3.0
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; as, " a citizen of no mean city," that is, of an illustrious city.
