In classical and medieval astronomy, what name was given to the outermost moving sphere in the geocentric model of the Universe?

In classical, medieval, and Renaissance astronomy, the Primum Mobile (or "first moved") was the outermost moving sphere in the geocentric model of the universe.

The concept was introduced by Ptolemy to account for the apparent daily motion of the heavens around the Earth, producing the east-to-west rising and setting of the sun and stars, and reached Western Europe via Avicenna.

The Ptolemaic system presented a view of the universe in which apparent motion was taken for real – a viewpoint still maintained in common speech through such everyday terms as moonrise and sunset. Rotation of the Earth on its polar axis – as seen in a heliocentric solar system, which (while anticipated by Aristarchus) was not to be widely accepted until well after Copernicus – leads to what earlier astronomers saw as the real movement of all the heavenly bodies around the Earth every 24 hours.

Astronomers believed that the seven naked-eye planets (including the Moon and the Sun) were carried around the spherical Earth on invisible orbs, while an eighth sphere contained the fixed stars. Motion was provided to the whole system by the Primum Mobile, itself set within the Empyrean, and the fastest moving of all the spheres.