The ATC ("Air Traffic Controller") is a tubular belay device made by Black Diamond. It became so dominant in the 1990s and 2000s that "ATC" has become climbing slang for any tubular belay device, regardless of brand — the same way "Hoover" means vacuum cleaner in the UK. The ATC works by passing the climbing rope through a slot, threading it around a carabiner clipped to the belayer's harness, and using friction between rope and metal to brake under load.
The ATC is mechanical: it has no moving parts and no auto-locking mechanism. The belayer must keep their brake hand below the device at all times to maintain friction; a hand off the rope can result in a dropped climber. Learning to belay correctly with an ATC is a non-negotiable skill — most climbing courses start here.
Newer alternatives like the Petzl GriGri are "assisted-braking" devices that lock under sudden load, providing a backup if the belayer's hand slips. Many gyms now require GriGris (or equivalents) for lead belaying. ATCs remain dominant for trad, multi-pitch, and rappelling because they're lighter, simpler, and work with any rope diameter. Most climbers own both.