A Sphere is one of ten divisions of the Tellurian. The divisions are largely artificial, but are fairly accurate in dividing the known universe into several distinct facets. Although the distinction was originally artificial, it's notable that each Sphere has its own associated Realm within the Umbra, matching up to one of the nine planets within the physical universe.
The Spheres have their origin in the court of Thothmes III and Hatshepsut, when they collected together sacred artisans and mystics from around the world into two organizations: the Reed of Djehuty and the Cupbearers of Aset. He declared nine Cornerstones of Creation, the building blocks of reality.
When the Reed and Cupbearers disbanded, the knowledge of the Cornerstones was lost for a time, with various fellowships utilizing their own Pillars and Ars which were completely incompatible with each other. In the late middle ages/early Renaissance, two seperate groups — the Order of Reason and the Council of Nine Mystic Traditions — arose which used these nine Cornerstones, renaming them Spheres.
The Spheres were:
By the modern nights, the Spheres are largely the same — although Connection has been renamed Correspondence.
Although Spheres are properly referred to using the capital S, modern Technocrats use the lower-case s when referring to their "spheres of reality." These spheres are treated as different scientific disciplines, like biology, physics, psychology or astronomy. Additionally, modern Technocrats do not practice Spirit, instead utilizing their own variant they've dubbed Dimensional Science.
Although the Nephandi still practice regular mystic Spheres, they also practice their own twisted versions known as Qlippothic Sphere. Each Sphere has its own associate Qlippoth: while normal Spheres operate by creating, changing and rearranging elements of reality, Qlippothic Spheres operate by reality's destruction. A normal portal might temporarily "sew" together two places in the Tapestry, but a portal created through Qlippothic magic would literally tear a hole in space, negating the space between two locations.
Mage: The Ascension Spheres