A geostationary satellite is so name because it a appears to be stationary to an observer on the earth. This satellite is also referred to as synchronous satellite because the angular speed of the satellite is synchronized with the angular speed of earth about the axis. When such a satellite is used for communication purposes, it is also known as communication satellite.
In order that a satellite may appear stationary to an observer on the earth, the following conditions must be satisfied.
It should be at a height of nearly 36,000 km above the equator.
The time taken by the satellite to complete one revolution around the earth is the same as that of the earth about its own axis i.e. 24 hours.