Plain English Meaning

A way of stating altitude in the upper airspace where every aircraft sets its altimeter to the same standard pressure. Once everyone is using the same setting, altitudes are written as flight levels — FL180 means 18,000 feet, FL350 means 35,000 feet — and pilots can be sure they are vertically separated from other traffic.

Full Definition

An altitude expressed in hundreds of feet, indicated on an altimeter set to the standard pressure of 29.92 inches of mercury (1013.2 hPa). For example, FL250 represents an indicated altitude of 25,000 feet on an altimeter set to 29.92. Flight levels are used at and above 18,000 feet MSL in the United States, where all aircraft use the standard altimeter setting so they share a common reference for vertical separation.