primum mobile

Meanings

1. The outermost celestial sphere of the heavens in Ptolemaic astronomy, which was believed to cause all the inner spheres to rotate.
2. The prime mover or first cause (“an initial cause from which all other causes and effects follow”).
3. The person or thing that is the main impetus for some action; a driving force.

Etymology

From Middle English primum mobile (“outermost celestial sphere causing other spheres to move”), borrowed from Medieval Latin prīmum mōbile (literally “first mover”), a calque of Arabic مُحَرِّك أَوَّل (muḥarrik ʔawwal, “first mover”), which is itself a calque of Ancient Greek τὸ πρῶτον κινῆσαν (tò prôton kinêsan, “prime mover”).