An Etymological Dictionary of Astronomy and Astrophysics
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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.

→ Keplerian; → disk.