understatement
n. C / Un. a statement that describes something as smaller or less important than it really is. You use it to be polite, funny, or to avoid sounding too dramatic.
n. a statement that represents something as smaller, less intense, or less important than it truly is. Often used as a rhetorical device to achieve irony or to maintain a modest tone.
Calling a hurricane 'a bit of rain' is a huge understatement.
To say the CEO was disappointed by the bankruptcy would be a massive understatement.
The author's use of dry understatement serves to heighten the emotional impact of the tragedy, as the characters' restrained reactions contrast sharply with the gravity of their situation.
From under- + statement or understate + -ment.
Often follows the verb 'to be' and is frequently modified by adjectives like 'massive', 'huge', or 'classic'.