A fusing piton was a metal spike which fused to rock in order to provide a stable anchor to attach climbing gear.
In 2369, Captain Jean-Luc Picard, Doctor Beverly Crusher, and Lieutenant Worf used fusing pitons while rappelling down a chasm on Celtris III. (TNG: "Chain of Command, Part I")
In 2373, Lieutenant Tom Paris, Lieutenant B'Elanna Torres, and Neelix used fusing pitons to descend down a small cliff while searching for gallicite. Neelix's piton, though used and set correctly, unexpectedly malfunctioned and caused him to become injured. (VOY: "Blood Fever")