A number denoting the experience level of a dreamer. The amount of XP required to advance in orbits increases exponentially as the avatar grows. Every ten orbits comprises a sphere.
As a dreamer progresses in orbits, they may unlock additional arts, which become available to learn from a teacher.
Arts have orbits as well, but they are progressed by practising the art. Every time an art is used, it has a chance to increase in orbit. An art's orbit cannot be any higher than the orbit of the dreamer.
Also see the XP and Sphere Table.