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.
See prime mover.
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.