The angular velocity of a point in a circular orbit around a central mass. It
is given by:
Ω_{K} = (GM/r^{3})^{1/2},
where G is the → gravitational constant, M is
the mass of the gravitating object, and
r is the radius of the orbit of the point around the object.

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.

The orbit of a spherical object of a finite mass around another
spherical object, also of finite mass, governed by their mutual
→ gravitational forces only.

A → rotation curve in which the speed of the orbiting body is
inversely proportional to the → square root of its distance
from the mass concentrated at the center of the system.

Shearing motion of an ensemble of particles, each on a nearly
circular, → Keplerian orbit.
→ Orbital velocity decreases as orbital radius increases,
yielding shear. Viscous drag on such shear, due to ring-particle collisions, plays a
key role in ring processes (Ellis et al., 2007, Planetary Ring Systems, Springer).