Chekhov's gun
A principle of narrative economy: every highlighted element must serve — the rifle on the wall must eventually fire.
Chekhov's gun states a principle of dramatic economy attributed to the Russian writer: if a rifle hangs on the wall in act one, it must fire before the end — otherwise it had no business being there. Every emphasized detail is a promise; every broken promise is dead weight misdirecting the reader's attention.
The principle works both ways. Downstream: don't emphasize what won't serve. Upstream: whatever must serve at the climax should be planted early and discreetly (the exact antidote to deus ex machina). The art lies in loading the gun without the reader realizing they were looking at a weapon — the difference between planting and telegraphing.
Example
A character's climbing hobby, mentioned in chapter 2, saves their life in chapter 30.