What Is TCAS? Definition & Meaning in Aviation
TCAS (traffic collision avoidance system) is an onboard system that warns pilots of nearby aircraft and tells them how to avoid a collision.
In plain language
It talks to the transponders of other aircraft. A traffic advisory (TA) warns of traffic; a resolution advisory (RA) tells the pilot to climb or descend.
Example
TCAS is why every airliner must carry a working transponder.
Why it matters
TCAS is an independent last line of defence against collisions. Its existence is one reason every airliner must carry a working transponder.