understatement for rhetorical effect, esp when achieved by using negation with a term in place of using an antonym of that term, as in "She was not a little upset" for "She was extremely upset".
Word origin
C17: from Greek, from litos small
litotes in American English
(ˈlaɪtəˌtiz; laɪˈtoʊtiz)
noun
understatement for effect, esp. such understatement in which something is expressedby a negation of the contrary (Ex.: not a few regrets)
Derived forms
litotic (liˈtotic) (laɪˈtɑtɪk)
adjective
Word origin
Gr litotēs < litos, smooth, simple, plain, akin to leios: see lime1