Altitude

[1] The height in feet or meters above sea level. For precision in determining the performance of an aircraft, this must be corrected for the deviation of the meteorological conditions from that of the International Standard Atmosphere.
[2] The angular distance of a heavenly body measured on that great circle which passes, perpendicular to the plane of the horizon, through the body and through the zenith. It is measured positively from the horizon to the zenith, from 0° to 90°.
[3] The line through the vertex of a geometrical figure or solid perpendicular to its base.
[4] The length of the line of definition #3.
See also cabin altitude, pressure altitude