Litotes
Etymology
From Greek litótēs, "plainness, simplicity," from litós ("plain, meager, small"). The name fits the effect: an understated, stripped-down way of asserting something. Rhetoricians classed it as a deliberate figure — modesty or irony achieved by negating the contrary rather than stating the claim directly.
noun · /LY-tuh-teez/
Affirming something by denying its opposite — saying "not bad" to mean good.
What it names
The dry, deflating register of praise-by-negation, which sounds more measured (and often more sincere) than saying the thing outright.
Example
Reviewing the sold-out show, she offered a bit of litotes: "You could do a lot worse on a Friday night."
Related idea
Understatement deflates by making something smaller than it is; litotes specifically works through negation — "not unimpressive" rather than merely "fine."