Joining in a single short expression two words of opposite meaning, such as "sweet bitterness".
In Inferno
but now extend your hand here; open my eyes. And I did not open them; and it was courtesy to be a villain to him.
"Courtesy" and "to be a villain" stand together in the same sentence: the nobility of conduct is joined to its opposite. The union of the two opposite words sums up Dante's gesture, which breaks the promise to a traitor and calls that gentleness.