Litotes

Litotes is understatement that affirms something by denying its opposite — 'not bad', 'no small achievement' — so the reader arrives at the positive meaning by way of the negative.

The reviews were not unkind. Two negatives resolving into a mild positive, and the mildness is doing something a direct statement could not: it suggests a speaker who is choosing not to say more.

Common forms: not bad, no small feat, hardly a disaster, not without merit, he was no fool.

Litotes is the engine of understatement, and understatement is often the stronger claim, because the reader supplies the missing intensity themselves and believes their own conclusion. The crossing was not comfortable, said by someone who has just described a storm, lands harder than the crossing was terrible.

In dialogue it is characterisation before it is anything else. A character who says I have had better weeks about a catastrophe is telling you how much they are willing to show, which makes litotes a natural fit for subtext and for oblique dialogue, where nobody says the thing directly.

Its opposite is hyperbole, and it is a mild relative of irony, since it also means more than it says.

Two limits. It needs a reader who can see the gap between the words and the situation, so the scene must supply the facts first. And a page of it reads as coy — the register only works because most sentences around it are direct.