System of Flight LevelsFlight level zero shall be located at the atmospheric pressure level of 1013.2hPa. Consecutive flight levels shall be separated by a pressure interval corresponding to at least 500ft (152.4M) in the standard atmosphere.
Source: ICAO
See also:
Altitude Regulations
Altimeter Setting
Transition Altitude
Transition Level
System of Flight Levels
Cruising Levels
Semicircular Cruising Level System (ICAO)
Cruising Levels Applicable in RVSM Airspace