litotes (LYE-tuh-teez) n. a rhetorical device by which understatement is achieved by denying a statement's opposite.
Derived from the Greek word λιτότης (litotēs), meaning "simple," this rhetorical effect is easier to demonstrate than to define. For example, I might say that a beautiful woman walking in front of me is "not unattractive" if I were desirous of avoiding my wife's elbow in my ribcage.
I was previously aware of the method, but until I came across litotes in a book called Beyond Realism: Turgenev's Poetics of Secular Salvation, I never bothered to look up the term.
I suppose one could say I became not uninterested in learning more about the meaning of litotes after reading the word.