Plain English Meaning
A runway or taxiway that has been shut down for good. The airport marks it with big yellow Xs so pilots know not to land or taxi on it.
Full Definition
Runways and taxiways that have been taken out of service permanently and are no longer usable for aircraft operations. They are identified by specific visual markings: lighting is disabled, runway threshold markings, runway designators, and touchdown zone markings are obliterated, and a yellow X is placed at each end of the runway and at 1,000-foot intervals along its length. Permanently closed taxiways are marked the same way at each end.
Context Anchor
Seen in airport marking and lighting guidance, on airport surfaces, and sometimes when looking at an airport from the air during or planning.