Suspension of disbelief
The pact by which readers accept a fiction's implausibilities — as long as the work respects its own rules.
The "willing suspension of disbelief," coined by poet Samuel Taylor Coleridge in 1817, names the reader's consent to switch their skepticism to standby: they know dragons don't exist and agree to believe in them for the story's duration, in exchange for fiction's pleasure.
That credit isn't unlimited, and its mechanics are counter-intuitive: readers forgive enormous premises (magic, faster-than-light ships) but not internal inconsistencies. A dragon doesn't break the pact; a character acting stupidly for the plot's convenience does. A fiction's credibility isn't played against reality, but against its own rules.
Example
We accept a world where the dead return; we don't accept a careful hero suddenly forgetting to lock his door.