litotes

In rhetoric, litotes is a figure of speech in which understatement is employed for rhetorical effect when an idea is expressed by a denial of its opposite, principally via double negatives. For example, rather than saying that something is attractive (or even very attractive), one might merely say it is "not unattractive."

Litotes is a form of understatement, always deliberate and with the intention of emphasis. However, the interpretation of negation may depend on context, including cultural context. In speech, it may also depend on intonation and emphasis; for example, the phrase "not bad" can be said in such a way as to mean anything from "mediocre" to "excellent."

The use of litotes appeals specifically to certain cultures including the northern Europeans and is popular in English, Russian, and French. They are features of Old English poetry and of the Icelandic sagas and are a means of much stoical restraint.

e.g. not bad means good.