Meaning of “understatement” - Learner’s Dictionary

understatement

noun [ C, U ] uk us /ˌʌndəˈsteɪtmənt/

a statement in which you say that something is less extreme than it really is:

'Quite big', did you say? That's an understatement - he's enormous!

(Definition of “understatement” from the Cambridge Learner’s Dictionary © Cambridge University Press)