a statement that is not strong enough to express how good, bad, impressive etc something really is:
To say the movie was bad is an understatement.
'It wasn't very easy to find the house.' 'That's got to be the understatement of the year!'
the practice of making something seem to have less of a particular quality than it really has
'We have not done quite well enough,' Macmillan said, with characteristic understatement.