A 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 Brothers Karamazov
God is only a hypothesis... but... I acknowledge that he is necessary, for order... for the order of the world and so on... and if he did not exist, it would be necessary to invent him.
The last clause repeats a famous line from books, that if God did not exist, one would have to invent him, which Ivan also quotes in the tavern. Kolya uses it without having thought it through, like someone showing off a borrowed object.
Book X: Ilyusha's death, Kolya Krasotkin to Alyosha, in the vestibule of the hut