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