(astronomy) the outermost empty sphere in the Ptolemaic system that was thought to revolve around the earth from east to west in 24 hours carrying with it the inner spheres of the planets, sun, moon, and fixed stars
"the first source of motion," mid-15c., from Medieval Latin (11c.), literally "the first movable thing;" see prime (adj.) + mobile. A translation of Arabic al-muharrik al-awwal "the first moving" (Avicenna).