litotes

n. (pl. Same) ironic understatement, esp. Using the negative (e.g. I shan't be sorry for i shall be glad). [greek litos plain, meagre]

n. 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.