Soft cap and hard cap
Also said as Diminishing returns, Stat cap, Level cap
A hard cap is a limit that can't be passed. A soft cap is a point past which each extra bit does less, so more is still allowed, and hardly worth buying.
Without limits, the best build pours everything into one stat, and at the extreme the stat breaks the game. Caps keep a good number from turning into an infinite one, and they push players to spread their points around.
Say it
“Crit chance needs a soft cap: full value to 40%, then half value to 60%, and a hard stop at 75%, so stacking it never makes other stats pointless.”
Where you’ve seen it
Path of Exile limits elemental resistances to 75% by default, a hard cap, since 100% would mean no damage at all. Elden Ring's Vigor gives far less health per point after 40, and less again after 60, a soft cap.
Not to be confused with level cap
A level cap limits how far your character can level. A stat cap limits one number. The same words get used for both, so say which you mean.