litotes \LAHY-tuh-teez, noun:

Understatement, especially that in which an affirmative is expressed by the negative of its contrary, as in “not bad at all.”

Stevens does not allow himself much of the Sublime here, yet it creeps in by negation in the litotes or understatement of the stanza’s close.
— Harold Bloom, Wallace Stevens: The Poems of Our Climate

I know it’s a textbook example of what lit-crit geeks like to call litotes, a figure of speech in which an affirmative is expressed through the negation of its opposite…
— Mark Dery, I Must Not Think Bad Thoughts

Litotes comes from the Greek word lītótēs which meant “plainness, simplicity.”