FL, or Flight Level, is a notation for indicated altitude
(divided by 100, or drop the last two zeros) when the altimeter is set to standard pressure, or 29.92 inches of mercury (1013 mb). In the continental U.S. flight levels are defined between 18,000 feet (FL180) and 60,000 feet (FL600). In other countries the flight levels may start a lot lower in some areas.
the flight level
page on Wikipedia.