understatement
noun [ C or U ]
uk/ˌʌndəˈsteɪtmənt/ usa statement that something is much smaller, much less important, or much less serious than it really is:
It's an understatement to say the workers are worried about their futures.
with understatement The report says with understatement, 'there is still much scope for reform'.
understatement of the century/decade/year To say he's been busy would be the understatement of the year.
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overstatement