Litotes

Litotes (Gr. λιτότης, plainness, λιτός, 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. μείωσις, lessening, diminution, μείων, lesser), where the expressions used apparently are weak or understated, but the effect is to intensify.