Astron. in the Ptolemaic system, the tenth and outermost concentric sphere, revolving from east to west around the earth and causing all celestial bodies to revolve with it
See primum mobile in American Heritage Dictionary 4
(prīˈməm mōˈbə-lēˌ, prēˈməm mōˈbĭ-lāˌ)
noun
The tenth and outermost concentric sphere of the universe thought in Ptolemaic astronomy to revolve around the earth from east to west in 24 hours and believed to cause the other nine spheres to revolve with it.