Official Aviation Definition
The Traffic Collision Avoidance System (TCAS) is an airborne collision avoidance system mandated on commercial air transport. By independently interrogating Mode C and Mode S transponders of nearby aircraft, TCAS generates Traffic Advisories (TA) and vertical Resolution Advisories (RA) directing pilots to climb or descend.
In Simple Words
An onboard radar safety system that watches for other nearby airplanes and gives emergency audio commands like "CLIMB! CLIMB!" if a mid-air collision is possible.
Real-World Flight Example
When the opposing jet breached vertical separation, TCAS commanded a Resolution Advisory: "DESCEND, DESCEND NOW."