litotes

‖n. [ NL., fr. Gr. lito`ths, from lito`s plain, simple. ] (Rhet.) 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; a form of understatement; as, “ a citizen of no mean city, ” that is, of an illustrious city; or, “not bad”, meaning “good”. [ 1913 Webster +PJC ]