Understatement for effect

• 'Not unhappy,' e.g
• Ironical understatement
• Literary understatement
• Figure of speech like "no mean feat"
• Figure of speech that employs ironic understatement
• "Not too bad" for "very good"
• using ___: “Mary had no giant lamb / Its fleece was not unwhite”

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 ...

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