A belay device is a mechanical device used in climbing, canyoneering, and mountaineering to control the rope during a fall. It allows a climber to ascend or descend safely while providing a means for the belayer to manage the rope tension. Common types of belay devices include tube devices, auto-locking devices, and figure-eight devices. These devices work by creating friction against the rope, which helps to catch a climber in case of a fall. Proper use of a belay device is essential for safety, making it a critical tool for both novice and experienced climbers.