Dynamic difficulty adjustment (DDA)
Lets a game read how the player is doing and quietly tune the challenge, keeping it balanced without a visible difficulty dial.
The system measures signals like deaths, time per room or hit rate, then adjusts enemy count, damage or resources. An early and often-cited example is Resident Evil 4 (2005), which secretly moved the player between hidden difficulty tiers. Left 4 Dead's AI Director does the same with pacing and spawns.
The criticism is that the rubber-band effect can feel unfair when the opposition visibly eases off the moment you succeed, and that it can undercut the player's sense of actually improving. Well hidden, though, it carries more players to the end.