Epicyclic frequency in an accretion disk is the
frequency at which fluid within the disk oscillates in
a circular (or oval) motion relative to a normal orbital motion.
The term is often used for such a radial oscillation (within the
plane, radial epicyclic frequency) but is also used for oscillation
out of the plane (vertical epicyclic frequency).
The term epicyclic frequency is similarly used in relation to the
orbits of stars around a galaxy, which typically similarly
oscillate relative to a circular orbital path.