Understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary) WordNet 3.0 & Open English WordNet

In rhetoric, litotes, also known classically as antenantiosis or moderatour, is a figure of speech and form of verbal irony in which understatement is used to emphasize a point by stating a negative to further affirm a positive, often incorporating double negatives for effect. Wikipedia

In rhetoric, meiosis is a euphemistic figure of speech that intentionally understates something or implies that it is lesser in significance or size than it really is. Wikipedia

Figure of speech Wikidata

Euphemistic figure of speech Wikidata

An ironic figure of speech whereby something is stated by denying its opposite, particularly the negation of a negative quality to say something positive. Wiktionary

Rhetoric: stating a point by denying its opposite. Wiktionary (translation)

A figure of speech whereby something is made to seem smaller or less important than it actually is; understatement. Wiktionary