Plain English Meaning
The runway length the airplane needs from the moment it crosses the runway edge at 50 feet up to the moment it stops rolling. It's the air portion plus the ground portion added together.
Full Definition
The total horizontal distance required for an airplane to land and come to a complete stop, measured from a point 50 feet above the landing surface (the threshold crossing height) to the point on the runway where the airplane stops. It includes the airborne distance from the 50-foot point to touchdown, plus the ground roll from touchdown to full stop.
Context Anchor
Used when checking whether a runway is long enough for the planned landing, especially in performance planning for airplanes.