Brief reference, without explanation, to a person, a text, a myth or a fact known outside the work, which carries with it the meaning of that other story.
In The Death of Ivan Ilyich
The syllogism example he had studied in Kiesewetter's logic, Caius is a man, men are mortal, therefore Caius is mortal, had seemed to him all his life correct only in relation to Caius, but in no way in relation to him.
Caius's "syllogism," "which he had studied in Kiesewetter's logic," points to the textbook example. The reference gives Ivan Ilyich the baggage of an educated man and gives the sentence the force of a school truth that is contradicted.