Plain English Meaning
An onboard system that watches for nearby aircraft and warns the pilot when one is getting too close, but only tells the pilot that traffic is there — it does not tell the pilot what to do about it.
Full Definition
TCAS I is the first-generation airborne collision avoidance system that monitors nearby transponder-equipped aircraft and issues Traffic Advisories (TAs) to alert the pilot to potential conflicts. TCAS I provides bearing, range, and relative altitude information for surrounding traffic but does not issue resolution instructions; the pilot must visually acquire the traffic and decide on any avoidance action.
Context Anchor
Seen in instrument flying, traffic displays, and discussions of traffic alert and collision avoidance equipment.