Plain English Meaning
Airports with a working control tower. Pilots must talk to the tower on the radio and follow its instructions for taxi, takeoff, landing, and flying in the pattern.
Full Definition
Airports that have an operating control tower, where air traffic controllers issue instructions and clearances to aircraft operating on the ground and within the surrounding airspace. Pilots are required to establish two-way radio communication with the tower and comply with ATC instructions before entering the traffic pattern, taxiing, taking off, or landing.