Litotes

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.

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"