litotes [Grk lītótēs ‘plainness, simplicity’]

Rhetorical trope which replaces a stressed, positive expression by the negation of the opposite, e.g. not (exactly) small=‘(rather) large.’ Litotes expresses understatement that intensifies meaning, as in He is not the wisest man in the world when we mean ‘He is a fool’ (Peacham, The Father of Eloquence, 1577).