Plain English Meaning
An altitude of 18,000 feet measured using a fixed altimeter setting (29.92) instead of the local one. It is the bottom of the high-altitude airspace where everyone must fly on an instrument flight plan.
Full Definition
Flight Level 180, an altitude of 18,000 feet referenced to the standard altimeter setting of 29.92 inches of mercury. FL 180 is the lowest flight level in the contiguous United States and marks the floor of Class A airspace, where all aircraft must operate under instrument flight rules with altimeters set to 29.92.
Context Anchor
Seen in instrument procedures, air traffic control clearances, , and discussions of U.S. high-altitude airspace.