Wallhack
Also called: WH, Walls
Any cheat that lets you see players through walls. In practice it means the same as ESP.
Wallhack is the older, more general name for seeing enemies through walls. Today it is used interchangeably with ESP, chams and glow.
The risk with walls is not the anti-cheat seeing your screen; it is your behaviour. Pre-aiming corners and tracking players through cover gets you reported. Use the information, but don't act on what you couldn't know.