Litotes

A rhetorical term designating a kind of understatement in which instead of explicitly making a statement one instead negates its opposite. The most common example of this would be using "not bad" for "good". It differs from meiosis (don't be confused by the biological sense) in that meiosis does not involve double negation, it is simple understatement for the purpose of irony. For instance, if I used litotes to refer to the Declaration of Independence I might say it was "not unimportant", while if I were using meiosis I would say it was "slightly important".

Li"to*tes (?), n. [NL., fr. Gr. , from plain, simple.] Rhet.

A diminution or softening of statement for the sake of avoiding censure or increasing the effect by contrast with the moderation shown in the form of expression; as, " a citizen of no mean city," that is, of an illustrious city.

© Webster 1913.