Litotes /(lī"tō̇*tēz)/

Li·to·tes

Litotes

n.
  1. 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”. (Rhet.)