using ___: “Mary had no giant lamb / Its fleece was not unwhite”

• Ironical understatement
• "Not too bad" for "very good"
• Figure of speech like "not unlike"
• Literary understatement
• Toilets revolting; and that's an understatement
• Figure of speech that employs ironic understatement
• Understatement for effect
• 'Not unhappy,' e.g

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Meiosis \Mei*o"sis\ (m[-i]*[=o]"s[i^]s), n. [NL., fr. Gr. mei`wsis, fr. meioy^n to make smaller, from mei`wn. See Meionite .] (Rhet.) Diminution; a species of hyperbole, representing a thing as being less than it really is; understatement; see also litotes ...