A literary element that, at first, seems to have no purpose but is later revealed to be
important to the plot. This is commonly thought of as a parallel of
foreshadowing, but it is more properly interpreted as "do not include any
unnecessary elements in a story."
The return of Carmelita Spats in The
Slippery Slope after her seemingly
unnecessary appearance in The Austere Academy is an example of Chekhov's gun.
"One must not put a loaded rifle on the stage if no one is thinking of firing it." -Anton Chekhov