The figure of speech that deliberately understates something through a negative expression such as “not bad” is litotes.
Litotes is a form of understatement that often affirms a positive by denying its opposite. Saying “she is not unkind” may imply that she is kind, while “that was no small achievement” suggests it was impressive. The effect is usually indirect, restrained, or modest rather than openly emphatic.
The term comes from the Greek word litótēs, meaning simplicity or plainness. The device appears in classical rhetoric and is especially associated with the restraint of Old English poetry, Icelandic sagas, and stoic styles of expression. Cicero used the word in the first century BCE, though its rhetorical meaning developed over time.
Litotes is easy to confuse with an ordinary negative or with irony. “The result was not unexpected” may simply mean expected, but context and emphasis determine whether it functions as deliberate understatement. “Not bad” can also mean merely adequate, so the speaker’s tone matters. Related terms such as meiosis and understatement overlap with litotes, but litotes is particularly recognizable through negation.