Litotes
LITOTES (Gr. XITOTTJS, plainness, XITOS, plain, simple, smooth), a rhetorical figure in which emphasis is secured for a statement by turning it into a denial of the contrary, e.g. " a citizen of no mean city," i.e. a citizen of a famous city, "A. is not a man to be neglected." Litotes is sometimes used for what should be more strictly called " meiosis " (Gr. ^oocrw, lessening, diminution, fitidiv, lesser), where the expressions used apparently are weak or understated, but the effect is to intensify.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)