Plain English Meaning
The line down the middle of the runway. Pilots aim to keep the airplane tracking along it during takeoff and landing.
Full Definition
The painted or imaginary line running lengthwise down the exact middle of a runway or taxiway, used as the primary directional reference for takeoff, landing, and ground operations.
Context Anchor
Seen during taxi, takeoff roll, landing, and any instruction to keep the airplane aligned with the runway.
Why Pilots Care
Keeping the aircraft on the centerline prevents runway excursions that could lead to loss of control, propeller strikes, or runway-edge damage.