Dictionary:

primum mobile

  (prī'məm mō'bə-lē', prē'məm mō'bĭ-lā')

1. 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.
2. See prime mover (sense 1).

[Medieval Latin prīmum mōbile : prīmum, neuter of prīmus, first + mōbile, from neuter of mōbilis, movable (translation of Arabic al-muḥarrik al-’awwal : al-, the + muḥarrik, mover + ’awwal, first).]

(Latin, first in movement) In Aristotelian cosmology the first or outermost sphere of the heavens. The perfect, spherical rotation of this sphere is directly activated by love of God; the motion is as like the life of God as movement can be. See mover, unmoved.

The noun has one meaning:

Meaning #1: a self-caused agent that is the cause of all things
  Synonyms: first cause, prime mover