The Collaborative International Dictionary of English v.0.48:

Understatement \Un"der*state`ment\, n.
   The act of understating, or the condition of being
   understated; that which is understated; a statement below the
   truth.
   [1913 Webster]

WordNet (r) 3.0 (2006):

understatement
    n 1: a statement that is restrained in ironic contrast to what
         might have been said [ant: exaggeration, magnification,
         overstatement]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "understatement":
   abstractionism, coloring, deformation, distortion, exaggeration,
   expressionism, false coloring, falsification, garbling, hyperbole,
   inaccuracy, injustice, litotes, miscoloring, misdrawing,
   mispainting, misquotation, misreport, misrepresentation,
   misstatement, misteaching, nonrealism, overdrawing, overstatement,
   perversion, quietness, restrainedness, restraint, simplicity,
   slanting, subduedness, twisting, unobtrusiveness