Litotes
Litotes (lī"to*tēz) , noun
[New Latin, from Greek lito`ths, from lito`s plain, simple.]
(Rhetoric) 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”.