Definition
A method of expressing altitude as a three-digit number representing hundreds of feet above the standard datum of 29.92 inches of mercury, used at and above 18,000 feet MSL in the United States. FL 250 means 25,000 feet, FL 260 means 26,000 feet, and FL 270 means 27,000 feet, with all aircraft at these altitudes referencing the same standard altimeter setting rather than local pressure.
Plain English
At higher altitudes, pilots stop using local pressure on their altimeter and switch to a single agreed setting. The altitude shown is then called a Flight Level. FL 250 is 25,000 feet, FL 260 is 26,000 feet, and FL 270 is 27,000 feet, when everyone is using that shared setting.
Context Anchor
You see flight levels in ATC clearances, IFR altitude assignments, and high-altitude route descriptions.
Derivation
The term comes from the idea of a 'level' of flight — a horizontal layer in the sky. Because everyone above 18,000 feet uses the same altimeter setting (29.92), aircraft at the same Flight Level are truly at the same indicated altitude relative to each other, even if local sea-level pressure is different.
Why Pilots Care
Flight levels keep aircraft separated vertically using one common pressure setting so changing weather does not create altitude conflicts.
Intuition Check
Do not read FL 250 as 250 feet. The number after FL is in hundreds of feet, so FL 250 means 25,000 feet on the standard pressure setting.
Example Sentence 1
Center cleared us to climb and maintain FL 260, so I set 29.92 in the altimeter and leveled off at 26,000 feet.