The Collaborative International Dictionary of English v.0.48:

Litotes \Li"to*tes\ (l[imac]"t[-o]*t[=e]z), n. [NL., fr. Gr.
   lito`ths, from lito`s plain, simple.] (Rhet.)
   A diminution or softening of statement for the sake of
   avoiding censure or increasing the effect by contrast with
   the moderation shown in the form of expression; a form of
   understatement; as, " a citizen of no mean city," that is, of
   an illustrious city; or, "not bad", meaning "good".
   [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Meiosis \Mei*o"sis\ (m[-i]*[=o]"s[i^]s), n. [NL., fr. Gr.
   mei`wsis, fr. meioy^n to make smaller, from mei`wn. See
   Meionite.]
   1. (Rhet.) Diminution; a species of hyperbole, representing a
      thing as being less than it really is; understatement; see
      also litotes.
      [1913 Webster +PJC]

   2. (Cell Biology) The cellular process by which a diploid
      progenitor cell forms haploid gametes, including a
      division of one diploid cell into two cells, each with one
      of the homologous sets of chromosomes.
      [PJC]

WordNet (r) 3.0 (2006):

litotes
    n 1: understatement for rhetorical effect (especially when
         expressing an affirmative by negating its contrary);
         "saying `I was not a little upset' when you mean `I was
         very upset' is an example of litotes" [syn: litotes,
         meiosis]