Plain English Meaning
Machines on the ground that act like an aircraft so pilots can train safely without leaving the hangar. Some are simple, some are very realistic, and the FAA decides what kind of training each one counts for.
Full Definition
Ground-based training equipment that reproduces aircraft systems, flight characteristics, and the operational environment to allow pilots to practice procedures and maneuvers without flying an actual aircraft. The category covers a range of devices from basic procedure trainers to full flight simulators, each approved by the FAA at a specific level that determines what training and credit it can be used for.