Suspension of disbelief
Also said as Willing suspension of disbelief
The player agrees to accept things that are not true, in exchange for a story. Samuel Taylor Coleridge named it in 1817.
It is a loan, not a gift. Players will accept one huge impossible thing with no fuss, and then walk out over a small careless one, because the small one shows nobody was paying attention.
Say it
“Nobody minds the dragons. They mind that the guards ignore you carrying a ladder through the throne room. Fix the guards and the dragons stay fine.”
Where you’ve seen it
Portal asks you to accept a gun that makes holes in space, then takes care over a smaller question: Chell wears springs on her heels, the reason a fall does not hurt her. Portal 2 calls them Long Fall Boots.
Not to be confused with verisimilitude
Suspension of disbelief is what the player does for you. Verisimilitude is what you do to make that easy.