'Understatement' definitions:

Definition of 'understatement'

From: WordNet
noun
A statement that is restrained in ironic contrast to what might have been said [ant: exaggeration, magnification, overstatement]

Definition of 'Understatement'

From: GCIDE
  • 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]