The notion of the "unmoved mover" (originating from the Ancient Greek phrase "ὃ οὐ κινούμενον κινεῖ" which translates to "that which initiates motion without being set in motion") or the "prime mover" (referred to as "primum movens" in Latin) was introduced by Aristotle as a fundamental cause, often described as the initial uncaused cause, or the impetus behind all motion within the cosmos.
Newton stated in his Principia: "Hactenus Phænomena cælorum & maris nostri per Vim gravitatis exposui, sed causam Gravitatis nondum assignavi. Oritur utique hæc Vis a causa aliqua quæ penetrat ad usque centra Solis & Planetarum, sine virtutis diminutione."
Transl.: So far, I have explained the phenomena of the heavens and our sea through the force of gravity, but I have not yet assigned the cause of gravity. Surely, this force arises from some cause that penetrates all the way to the centers of the Sun and planets without a decrease in its power.