primum mobile
noun
pri·mum mo·bi·le ˈprī-məm-ˈmō-bə-lē 
ˈprē-
plural primum mobiles
: the outermost concentric sphere conceived in medieval astronomy as carrying the spheres of the fixed stars and the planets in its daily revolution
Word History
Etymology
Middle English, borrowed from Medieval Latin prīmum mōbile, literally, "first moving thing" (as translation of Arabic al-muḥarrik al-awwal) from Latin prīmum, neuter of prīmus "first, earliest" + Medieval Latin mōbile, noun derivative from neuter of Latin mōbilis "moving quickly, shifting" — more at prime entry 2, mobile entry 1
First Known Use
15th century, in the meaning defined above