Traffic Alert Collision Avoidance System (TCAS)

An airborne collision avoidance system based on radar beacon signals which operates independent of ground-based equipment. TCAS-I generates traffic advisories only. TCAS-II generates traffic advisories, and resolution (collision avoidance) advisories in the vertical plane. (PCG definition)

An airborne system developed by the FAA that operates independently from the ground-based Air Traffic Control system. Designed to increase flight deck awareness of proximate aircraft and to serve as a “last line of defense” for the prevention of midair collisions. (PHAK definition).

Source: PCG, Pilot Handbook of Aeronatical Knowledge