Definitions for "Geopotential height"
The height of a given point relative to sea-level, obtained by dividing the geopotential by a constant. See Appendix D.
The height above sea level of a pressure level. For example, if a station reports that the 500 mb height at its location is 5600 m, it means that the level of the atmosphere over that station at which the atmospheric pressure is 500 mb is 5600 meters above sea level. This is an estimated height based on temperature and pressure data.
The altitude of a layer in the atmosphere. It is expressed in geopotential meters and is equal to g/9,8 times the geopotential height expressed in (geometric) meters, g being the local acceleration of gravity. It is used to define isobaric surfaces on upper level charts. Reason for using geopotential meters instead of meters is to compare heights in geographical context, without the effect of gravity, thus imitating a perfect round earth.