TCAS

The Traffic alert and Collision Avoidance System (or TCAS) is a computerised avionics device which is designed to reduce the danger of mid-air collisions between aircraft. It monitors the airspace around an aircraft, independent of air traffic control, and warns pilots of the presence of other aircraft which may present a threat of mid-air collision (MAC). It is an implementation of the Airborne Collision Avoidance System mandated by International Civil Aviation Organization to be fitted to all aircraft over 5700 kg or authorised to carry more than 19 passengers.
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Tricyclic antidepressants (abbreviation TCA) are a class of antidepressant drugs first used in the 1950s. They are named after the drugs' molecular structure, which contains three rings of atoms (compare tetracyclic antidepressant).
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Utilizes interrogations of, and replies from, airborne radar beacon transponders and provides traffic advisories to the pilot. (14CFR1)

Utilizes interrogations of, and replies from airborne radar beacon transponders and provides traffic advisories and resolution advisories in the vertical plane. (14CFR1)

Utilizes interrogation of, and replies from, airborne radar beacon transponders and provides traffic advisories and resolution advisories in the vertical and horizontal planes to the pilot. (14CFR1)

Traffic Alert And Collision Avoidance System
  

(See TRAFFIC ALERT AND COLLISION AVOIDANCE SYSTEM.)