Joining in one short expression two words of opposite meaning, such as "bitter sweetness".
In Purgatory
so early did my Nella lead me to drink the sweet absinthe of martyrdoms, with her weeping undone. With her devout prayers and her sighs she took me from the slope where one waits, and freed me from the other turns.
"Sweet absinthe" joins the sweet and the bitter in the same expression. The pain is bitter, and it is sweet because it was Nella's weeping that brought him there; the word holds the pain and the gratitude at once.