Figure of Speech
Litotes
From Greek litotes: plainness. Common in Old English poetry ('not at all small' for 'enormous'); a characteristic of Anglo-Saxon understatement and modern British irony.
Definition
Understatement achieved by negating the opposite of what is meant, typically for ironic or emphatic effect.
Example
'He's not the brightest bulb'; 'Not bad' as high praise; 'The earthquake caused not inconsiderable damage.'