'Not unhappy,' e.g

• using ___: “Mary had no giant lamb / Its fleece was not unwhite”
• Figure of speech like "not unlike"
• Figure of speech like "no mean feat"
• Little child in stories making understatement
• Literary understatement
• Understatement for effect

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

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
rhetorical figure in which an affirmative is expressed by the negative of its opposite, from Greek litotes , literally "plainness, simplicity," from litos "smooth, plain, small, meager," from PIE root *(s)lei- "slimy, sticky, slippery" (hence "smooth"); ...

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 ...

Wikipedia Word definitions in Wikipedia
In rhetoric , litotes (, or ) is a figure of speech wherein understatement is used to emphasize a point by stating a negative to further affirm a positive, often incorporating double negatives for effect. For example, "She's not bad looking" could be used ...