Definition of primum mobiles:
(n) :
(by extension) The person or thing that is the main impetus for some action; a driving force.
(n) :
(chiefly philosophy, theology) The prime mover or first cause (“an initial cause from which all other causes and effects follow”).
(n) :
(astronomy, historical) The outermost celestial sphere of the heavens in Ptolemaic astronomy, which was believed to cause all the inner spheres to rotate.