A theorized 10th Sphere of Sphere magick. Mages have different theories of what it is but have trouble proving them. Some believe understanding the 10th Sphere will open a full understanding of the forces that govern the universe. Many Technocrats believe it is the Grand Unification Theory of physics. Pg. 68 of 2nd ed core discusses it.
As time went on this became increasingly a misnomer. The 10th Sphere of magick started out as a theorized 10th Sphere. A Sphere waiting to be discovered and added to the existing 9. It was an acknowledgement that even though the fairly new idea of 9 Spheres of magic was good, it might have missed something. The faction of mages that discovered it and learned to create Effects with it would garner influence and prestige among the Council of 9.
It quickly started morphing into something else. "10th Sphere" became a name for a breakthrough new understanding of all magick. It became a framework that would give new insight to magick and allow a mastery of it that wasn’t possible before.