Litotes

Litotes is a figure of speech that expresses an affirmative by negating the opposite. For example, the sentence, “The lady was not unattractive,” meaning that she was in fact attractive, is litotes. It derives from the Greek litotes meaning frugal or meagre in the sense that frugal or meagre words are used to express the opposite. Litotes as a figure of speech dates in the English language from the mid-17th century.