un‧der‧state‧ment /ˌʌndəˈsteɪtmənt $ -dər-/ noun
[Word Family: verb: state, understate ≠ overstate; noun: statement, understatement ≠ overstatement; adjective: understated ≠ OVERSTATED]

1. [countable] 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!’

2. [uncountable] the practice of making something seem to have less of a particular quality than it really has
with understatement
‘We have not done quite well enough,’ Macmillan said, with characteristic understatement.

[TahlilGaran] Dictionary of Contemporary English ▲

understatement
noun
ADJ. gross ‘A little strange’ is a gross understatement.
PHRASES a bit of/something of an understatement, the understatement of the year (informal) To say he wasn't amused must be the understatement of the year.

[TahlilGaran] Collocations Dictionary ▲