Verisimilitude
Also said as The appearance of truth, Believability
The quality of seeming true, which is not the same as being true. Small, specific details make a made-up thing feel as if it has been observed.
Players cannot check your world against reality, so they judge it by how sure of itself it sounds. One odd specific detail convinces more than a page of accurate summary.
Say it
“Do not research the real price of grain. Just give each shopkeeper one specific complaint, like the mule that bit her or the tax she refuses to pay, and they will read as real.”
Where you’ve seen it
In Red Dead Redemption 2, Arthur's hair and beard keep growing, and camp life goes on around you, chores and all. None of it is needed for the plot, and it is why people believe in the place.
Not to be confused with realism
Realism is about matching the facts. Verisimilitude is about feeling right, and it sometimes means getting the facts wrong: a film gunshot sounds nothing like a real one, and people believe the film.