n: a bit placed on a special device that serves to guide the device into an already existing hole that is to be opened (made larger in diameter). The pilot bit merely guides, or pilots, the cutters on the hole opener into the existing hole so that the hole-opening cutters can enlarge the hole to the desired size.

a small bit that drills a first hole to guide a larger drill

a small drill bit located in the center of the hollow core drill bit, that keeps the bit centered while starting a hole