Litotes
A figure that says less to suggest more, often by negating the opposite: "not bad" for "excellent."
Litotes asserts an idea by understating it, most often by negating its opposite: "not bad" (excellent), "no fool" (sharp). The reader reconstructs the real intensity — and that reconstruction makes the statement stronger than a direct assertion.
It is a figure of restraint: pudeur, polite irony, classical elegance. It is precious for characters who don't pour their hearts out, and for emotional scenes where underplaying touches more than pathos. Not to be confused with euphemism, which softens to spare; litotes pretends to soften in order to intensify.
Example
"He's not the friendliest person I know" (he is openly hostile).