|Definition - Using two forms of negation in the same clause or sentence.|
(1) When the double negative intensifies the negation, the phenomenon is called negative concord, e.g. "I ain't no loveboy" (= I'm not a loverboy).
(2) When the double negative cancels the negation, the phenomenon is called litotes, e.g. "He's not unkind" (= He's sort of kind).