A semi-synchronous orbit is an orbit with a period equal to half the average rotational period of the body being orbited, and in the same direction as that body's rotation.
For Earth, a semi-synchronous orbit is considered a medium Earth orbit, with a period of just under 12 hours. For circular orbits, the altitude is approximately 20,200 km.
See also 
- ^ NASA Technical Standard 8719.14 (draft) (PDF). NASA Orbital Debris Program Office. 8 Aug 2006.
- ^ "Catalog of Earth Satellite Orbits". 6 Sep 2012.