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 Earth orbits, the altitude is approximately 20,200 kilometres (12,600 mi).
^"NASA Technical Standard 8719.14 (draft)". NASA Orbital Debris Program Office. 8 Aug 2006. Archived from the original (PDF) on 2006-08-23. Cite journal requires |journal= (help)CS1 maint: discouraged parameter (link)