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 Master and Margarita
I witnessed all this personally. I was on the veranda with Pontius Pilate, in the garden, when he spoke with Caiaphas; I was also on the rostrum, only secretly, incognito
The names of Pontius Pilate and Caiaphas, spoken without introduction, bring into the conversation at Patriarch's Ponds the trial of Jesus that the foreigner has been narrating. Berlioz recognizes the reference and takes it for the delusion of someone who claims to be a witness.
Part One, Chapter 3: The seventh proof, The foreigner (Woland)