Altitude is the elevation of an object from a known level or datum, called zero level. Most often this level is defined as the absolute sea level, but it can vary.
In aviation, the term altitude is used to describe elevation above mean sea level, as opposed to height (which refers to elevation above a ground reference point) or flight level (the elevation according to a standard pressure altimeter setting).
In North America and the UK altitude is usually measured in feet. In continental Europe and other parts of the world altitude is measured in metres.
