An altitude of constant atmospheric pressure related to a reference datum of 29.92 inches of mercury, stated in three digits in hundreds of feet. In the US, the FL system is used for flights at and above 18,000 ft msl.
Altitude expressed in hundreds of feet. FL 350 is equal to 35,000 feet. Below about 20,000 feet, Flight Levels are not used. Military and some older civilian pilots may give their altitude in Angels. An Angel is equal to 1,000 feet. A pilot may also report his altitude as "level at 18,000". This is not the same as a Flight Level, but means that he has stopped climbing or descending at the indicated altitude.