Mental model
Also said as What the player thinks is going on, Player's theory of the game
The picture a player builds of how the game works. It is only ever a guess, and it steers everything they try next.
Players act on their model, not your code. If they believe that hiding in grass makes them invisible, your stealth system is being judged against a rule you never wrote. Don Norman's The Design of Everyday Things is the usual reference: the design has to communicate the model, because you won't be there to explain.
Say it
“Testers think the shield blocks everything, but it only blocks the front. Show a visible arc around the player when blocking and a spark on any blocked hit, so the model comes from the screen.”
Where you’ve seen it
In Super Mario Bros. the first Goomba walks straight at you, so you find out what enemies do, and the first mushroom slides off a pipe towards you, so you find out mushrooms help.
Not to be confused with tutorial
A tutorial is one way of teaching the model. The model is what the player actually ended up believing, which you only discover by watching them play.