Plain English Meaning
It is how air traffic control sees where aircraft are. Using radar and similar systems, controllers watch each aircraft's position so they can keep them safely apart and help them get where they are going.
Full Definition
The function performed by air traffic control to track aircraft positions using radar, ADS-B, or other approved surveillance systems, allowing controllers to monitor traffic, provide separation, issue traffic advisories, and offer navigational assistance.
Context Anchor
Seen in risk management, traffic advisory, flight following, and instrument flight discussions where a may receive help from air traffic control.