Definitions

Altitude

The vertical distance of an object measured from mean sea level.

Flight Level (FL)

A surface of constant atmosphere pressure which is related to a specific pressure datum, 1013.2hPa, and is separated from other such surfaces by specific pressure intervals.

Altitude above sea-level in 100 feet units measured according to a standard atmosphere. Strictly speaking a flight level is an indication of pressure, not of altitude. Only above the transition level (which depends on the local QNH but is typically 4000 feet above sea level) are flight levels used to indicate altitude; below the transition level feet are used.

e.g. FL250 = 25,000 feet above mean sea level when the pressure at sea level is 1013.2 mb.

Elevation or Height

The vertical distance of a level, a point, or an object considered as a point, measured from a specified datum.