Traffic collision avoidance system

A traffic alert and collision avoidance system (TCAS; TEE-kas), also called an airborne collision avoidance system (ACAS), is an aircraft collision avoidance system designed to reduce the incidence of mid-air collision (MAC) between aircraft. It monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control, and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of MAC. It is a type of airborne collision avoidance system mandated by the International Civil Aviation Organization to be fitted to all aircraft with a maximum take-off mass (MTOM) of over 5,700 kg (12,600 lb) or authorized to carry more than 19 passengers.

Source: Wikipedia — Traffic collision avoidance system (CC BY-SA 4.0)