litotes

noun

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

   noun.communication

   Examples

   Saying `I was not a little upset' when you mean `I was very upset' is an example of litotes.

   Synonyms meiosis

   Source: WordNet 3.0

2. 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.

   noun

   Source: webster1913