Definition
A surface of constant atmospheric pressure that is referenced to the standard pressure datum of 1013.2 hectopascals (29.92 inches of mercury), separated from other such surfaces by specific pressure intervals. Flight levels are expressed as a three-digit number representing hundreds of feet — for example, FL250 corresponds to a pressure altitude of 25,000 feet when the altimeter is set to the standard datum.
Plain English
A standard altitude used at higher levels of flight. Every aircraft sets its altimeter to the same agreed reference (1013.2 hPa / 29.92 inHg), so they all measure height the same way. FL250 means roughly 25,000 feet on that shared setting.
Context Anchor
Seen in air traffic clearances, flight plans, position reports, and high-altitude operations where aircraft use the standard pressure setting.
Derivation
The phrase combines 'flight' with 'level' in the sense of a horizontal layer. It came into use to describe a pressure-based layer in the sky rather than a true altitude above the ground or sea, because at high altitudes a shared pressure reference keeps all aircraft vertically separated even when local pressure varies.
Why Pilots Care
Provides consistent vertical separation between aircraft when local pressure changes would otherwise make true altitudes differ.
Analogy
It is like everyone agreeing to measure from the same zero mark on a ruler. The actual ground below may rise or fall, but all aircraft using flight levels are measuring from the same shared reference.
Grounding Statement
When aircraft use flight levels, their altimeters are set to the same standard pressure so their indicated levels match each other consistently.
Intuition Check
Flight Level does not mean the airplane is flying level, and it does not mean exact height above the ground. It means a pressure-based level using the standard pressure reference.
Example Sentence 1
The crew was cleared to climb to FL310 and reset the altimeter to the standard pressure setting passing the transition altitude.
Example Sentence 2
At flight level one eight zero we reset the altimeter to standard pressure and continued the cruise.