stands for Geosynchronous Earth Orbit, aka GSO
aka Clarke Orbit
, cf LEO
As such it is a bit redundant, as Geo refers to the earth, see Geoid
; as does Earth.
It is the orbit above the equator rotating in the same direction as the earth that takes exactly one sidereal day to rotate; this means an object placed there does not move relative to the earths surface. This is used extensively for Satellite communications, especially television.