Plain English Meaning

On low-pressure days, the bottom of the flight-level system moves upward. This term is the lowest flight level a pilot is allowed to use given the current pressure conditions, so that aircraft above 18,000 feet stay safely separated from aircraft flying altitudes referenced to local pressure below them.

Full Definition

The lowest flight level available for IFR cruising flight, determined by the current altimeter setting in the area. When the altimeter setting is 29.92 inches of mercury or higher, FL180 is the lowest usable flight level. As the altimeter setting drops below 29.92, the lowest usable flight level rises in steps to ensure aircraft at least 18,000 feet of above when flying at the lowest available flight level.