Progression
Also said as Player progression, Gating, Unlocks
The way a game changes as you play, giving you more to do and more of it within reach: new abilities, new places, stronger numbers.
It gives the player a reason to think beyond the next minute. A locked door you have already seen is a promise, and it keeps pulling you back through the game.
Say it
“Put a locked door in the very first room, in plain view. The key arrives in hour two, and I want the player to remember that door the second they get it.”
Where you’ve seen it
Super Metroid is laid out around this: each new power, such as the Morph Ball, turns a gap you walked past hours ago into a way forward.
Not to be confused with difficulty curve
Progression is what you gain. A difficulty curve is what the game demands. Good games raise both together, so the gain does not make you bored and the demand does not make you quit.