A brief reference, without explanation, to a person, a text, a myth, or a fact known outside the work, which brings with it the meaning of that other story.
In Dom Casmurro
a handkerchief was enough to kindle Othello's jealousy and compose the most sublime tragedy in this world
"A handkerchief" and "Othello" point to Shakespeare's tragedy without explaining it: the name and the object are enough for the reader to link Bentinho's jealousy to the Moor's.
Chapters CXXV to CXXXV: The Resemblance and Othello, the narrator