understatement
US /ˌʌn.dɚˈsteɪt.mənt/
UK /ˌʌn.dɚˈsteɪt.mənt/
1.
the presentation of something as being smaller, worse, or less important than it actually is
:
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To say it was difficult would be an understatement.
•
His calm reaction was a huge understatement of his true feelings.