primus motor
/ˌpriːməs ˈməʊtɔː/ /ˌprʌɪməs ˈməʊtə/Now rare
noun
The original source of motion or action; the cause, instigator, prime mover.
Origin
Late 16th century; earliest use found in Thomas Lodge (1558–1625), author and physician. From post-classical Latin primus motor, lit. ‘first mover’ from classical Latin prīmus + mōtor.