litotes vs understatement

litotes and understatement both are nouns.

Word	Noun	Adjective	Verb	Adverb
litotes	Yes	No	No	No
understatement	Yes	No	No	No

As nouns, understatement is a hypernym of litotes; that is, understatement is a word with a broader meaning than litotes:

litotes: understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary)
understatement: a statement that is restrained in ironic contrast to what might have been said

Other hypernyms of litotes include rhetorical device.

litotes (noun)	understatement (noun)
understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary)	a statement that is restrained in ironic contrast to what might have been said