Litotes
WordNet
noun
(1) Understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary)
"Saying `I was not a little upset' when you mean `I was very upset' is an example of litotes"
WiktionaryText
Etymology
From from .
Noun
- A figure of speech in which the speaker emphasizes the magnitude of a statement by denying its opposite; a figure of speech in which understatement is used with negation to express a positive attribute; a form of irony