Plain English Meaning

It is the size of the protected zone around the runway that a pilot must stay inside while circling to land from an instrument approach. Stay inside the circle and at or above the published altitude, and you are guaranteed to clear obstacles.

Full Definition

The protected radius of airspace measured from each runway threshold within which an aircraft conducting a circling instrument approach is guaranteed obstacle clearance, provided the aircraft remains at or above the published circling minimum descent altitude. The size of this radius depends on the aircraft's approach category (A through E), and under newer , on the circling altitude itself, with larger radii applied to faster categories and higher .