Traffic Collision Avoidance System. It picks up on radar transponder returns from nearby aircraft, shows their relative position and altitude on a display in the cockpit, and warns the pilot when other aircraft are too close either vertically or horizontally. The more sophisticated ones can tell the pilot whether to climb or dive to avoid a collision.
}:-)4
When the TCAS says "Dive! Dive!", is it followed by a klaxon making one of those "AH-OOOGAHHHH" like a sub's diving alarm?
(Sorry, couldn't resist! I'm a pilot and an ex-submariner.)