Litotes

    In rhetoric, litotes is a figure of speech in which the speaker either strengthens or weakens the emphasis of a
    claim by denying its opposite. The literal meaning of a litotes is "not X (but not necessarily Y)", and a litotes can
    be used as an understatement, actually meaning "very much Y," or to express ambivalence. Like many figures of
    speech, litotes is better understood in some cultural and linguistic contexts than in others.


    Examples
    Litotes:

    "[…] no ordinary city." Acts 21:39 (NIV)
    "That [sword] was not useless / to the warrior now." (Beowulf)
    "O Oedipus, unhappy Oedipus!" (Oedipus the King)
    "He was not unfamiliar with the works of Dickens."
    "The food was not bad."
    "Reaching the moon was no ordinary task."
    "That was no big deal."
    "Don't fail me now!"
     As opposed to:

    "[…] an impressive city."
    "The sword was useful."
    "O miserable Oedipus!"
    "He was well acquainted with the works of Dickens."
    "The food was good."
    "Reaching the moon was a fantastic task."
    "That was nothing."
    "Help me!"

    Related phenomena
    In English, such expressions as "not not good" are commonly used colloquially to express ambivalence. This is
    not necessarily the opposite of a conventional litotes, where the intent would be to emphasize the magnitude of
    the goodness. In colloquial speech, "It's not good, but it's not not good" likely means something like, "It's not
    particularly good, but it's not particularly bad, either."

    In many languages, including some non-standard dialects of English, double negatives can be used to express a
    simple negation; for example, in Spanish, "No quiero nada" (literally "I don't want nothing") means "I don't want
    anything."

    In some languages, some litotes are so idiomatic that they are not true figures of speech, but rather fixed
    expressions. For instance, in French, the sentence "Il faut qu'il aille" means "It's necessary that he go", while its
    opposite, "Il ne faut pas qu'il aille" means "it is necessary that he not go", which is much stronger than its
    English counterpart.

    Retrieved from "http://en.wikipedia.org/wiki/Litotes"
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