An Etymological Dictionary of Astronomy and Astrophysics

A circumstellar disk (such as an → accretion disk or a → protoplanetary disk) in which the → angular velocity at each radius is equal to the angular velocity
of a circular → Keplerian orbit at the same radius. The
main characteristic of the Keplerian disk is that
→ orbital velocity
varies as r^-1/2. This means that an object on an orbit closer to the central mass turns more rapidly than that on a farther orbit. This velocity difference is at the origin of internal friction or kinematic viscous forces
between disk particles, which heats up the material.

See also: → Keplerian; → disk.