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1. expressing with restraint or lack of emphasis, usually used for an ironic' or rhetorical effect.

2.basically a figure of speech that writers use to subtly make a situation less important or serious than it really is.
Saying "We've had a little rain," when the neighborhood is flooded. Now that is an understatement.

The last line of Frost's "Birches" illustrates understatement: "One could do worse than be a swinger of birches."
by AuthenticJudge March 28, 2010
to state to weakly, or without emphasis
To say George W. Bush is a bad president would be an understatement

by Garrett San August 01, 2007
To say that such phrase or sentence is not well defined and/or weak in its own definition
Person A: I think Richard Simmons is gay.

Person B: That's an understatement.
by ___the_underscore___ April 05, 2010