Plain English Meaning

FL 240 means the aircraft is flying at what its altimeter reads as 24,000 feet, with every aircraft up there using the same standard pressure setting so they all measure altitude the same way.

Full Definition

FL 240 is a flight level corresponding to an indicated altitude of 24,000 feet when the altimeter is set to the standard pressure setting of 29.92 inches of mercury (1013.2 hPa). Flight levels are used at and above 18,000 feet MSL in the United States to provide consistent vertical separation between aircraft regardless of local pressure variations.

Context Anchor

Seen in instrument clearances, route planning, and en route for aircraft operating in the flight levels.