Litotes

In rhetoric, litotes ( /ˈlaɪtətiːz/, /ˈlɪtətiːz/ or /laɪˈtoʊtiːz/) is a figure of speech in which understatement is employed for rhetorical effect, principally via double negatives. For example, rather than saying that something is attractive (or even very attractive), one might merely say it is "not unattractive".