Chekhov’s gun
Etymology
An eponym from the Russian playwright Anton Chekhov, who advised in letters around 1889 that a rifle hung on the wall in the first act must be fired by a later one, or else it should not be there.
noun phrase · /CHEK-ofs gun/
The principle that every element introduced in a story must eventually matter.
What it names
A dramatic rule of economy — if a detail is shown, it must pay off — and a sharp lens for reading fiction, noticing planted objects that promise to fire later.
Example
The odd mention of a spare key in chapter two turns out to be Chekhov’s gun — it opens the locked room at the climax.
Related idea
Do not confuse it with a red herring, a detail planted precisely to mislead; Chekhov’s gun insists planted details pay off, while a red herring exists to make you expect the wrong payoff.