Constraint
Also said as Limitation, Design limit
A limit you put on the player on purpose: little ammo, one button, a clock, a small map. It takes away an easy answer so the player has to find a better one.
Unlimited options produce one dominant habit and no thought. Limits are what make choices cost something, and a good limit is often the identity of a game.
Say it
“Cap the player at two bullets, refilling one every 1.4 seconds. I want them waiting for enemies to line up, not holding down fire.”
Where you’ve seen it
In Superhot, time moves only when you move. That one limit turns a shooter into something closer to a puzzle.
Not to be confused with difficulty
Difficulty asks you to do a thing better. A constraint changes which thing you should be doing.