Definition
A level of constant atmospheric pressure related to a reference datum of 29.92 inches of mercury (1013.2 hPa). Flight levels are expressed in three digits that represent hundreds of feet, with the altimeter set to 29.92. For example, Flight Level 250 represents an altimeter indication of 25,000 feet.
Plain English
An altitude expressed as a three-digit number, used at higher altitudes where every aircraft sets the same standard pressure on the altimeter so they all share the same reference. FL250 means 25,000 feet on an altimeter set to 29.92.
Context Anchor
Used in high-altitude flying, air traffic control clearances, flight plans, and weather or route information where aircraft are operating on the standard pressure setting.
Derivation
Called a 'level' rather than an 'altitude' because at high altitude, pilots stop adjusting the altimeter to local pressure and instead share one standard pressure setting. The number marks a pressure layer in the atmosphere, not a true height above sea level.
Why Pilots Care
Using the correct flight level keeps aircraft vertically separated even when local barometric pressures differ, directly reducing the risk of mid-air collision.
Analogy
It is like everyone agreeing to measure from the same ruler instead of each person using a different starting point. The aircraft may be over areas with different weather, but the height reference stays the same.
Intuition Check
Do not read “level” as meaning the airplane is not climbing or descending. In “Flight Level,” it means a standard-pressure height reference, such as FL180 or FL350.
Example Sentence 1
ATC cleared the aircraft to climb and maintain Flight Level 350.
Example Sentence 2
Once above the transition altitude the crew reset the altimeter to 29.92 and leveled at flight level one eight zero.