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).
Toomre Q Parameter (Q)