The Collaborative International Dictionary of English v.0.48:

Understate \Un`der*state"\, v. t.
   To state or represent less strongly than may be done
   truthfully.
   [1913 Webster]

WordNet (r) 3.0 (2006):

understate
    v 1: represent as less significant or important [syn:
         understate, minimize, minimise, downplay] [ant:
         amplify, exaggerate, hyperbolise, hyperbolize,
         magnify, overdraw, overstate]