'Not unhappy,' e.g

• Figure of speech like "no mean feat"
• Little child in stories making understatement
• using ___: “Mary had no giant lamb / Its fleece was not unwhite”
• Understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary)
• Figure of speech like "not unlike"
• Toilets revolting; and that's an understatement
• "Not too bad" for "very good"

Wiktionary Word definitions in Wiktionary
n. (context rhetoric English) A figure of speech in which the speaker emphasizes the magnitude of a statement by denying its opposite; a figure of speech in which understatement is used with negation to express a positive attribute; a form of irony

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 ...