Litotes

In rhetoric, litotes (/ˈlaɪtətiːz/, US /ˈlɪtətiːz/ or /laɪˈtoʊtiːz/) 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 to express that someone is gorgeous. Or it could convey that she's not particularly ugly, but also isn't particularly attractive. The degree of emphasis depends on the context in which it is used. For instance, the commonly used phrase "not bad" could indicate that something was either average or excellent. Along the same lines, litotes can be used to diminish the harshness of an observation; "He isn't the cleanest person I know" could be used as a means of indicating that someone is a messy person.