Definition
TRAFFIC ALERT AND 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.
Source: FAA Pilot/Controller Glossary (verbatim from the Federal Aviation Regulations).