Litotes --
the opposite of hyperbole -- is a figure of speech in which deliberate
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." A good idea may be described as “not half bad,” or a difficult task considered “no small feat.”
As a form of deliberate understatement, litotes --
pronounced as LYE-tuh-teez -- always has the intention of placing emphasis. However, the negative or positive interpretation of this emphasis 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."
Litotes is found frequently in Old English poetry; “That was a good king,” declares the narrator of the Beowulf epic after summarizing the Danish king’s great virtues.
See also
Irony.