An Etymological Dictionary of Astronomy and Astrophysics
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The act of corotating.

Verbal noun of → corotate.

1) In the → X-wind model of → accretion, the distance from the star where the → centrifugal force on a particle corotating with the star balances the → gravitational attraction; in other words, where the → accretion disk rotates at the same → angular velocity as the star.
2) In a → spiral galaxy, the place where the spiral → pattern speed has the same velocity as the → rotation curve of the → galactic disk. In the frame rotating with the wave, particles inside this radius will appear to revolve in the direction of the frame rotation (prograde) while outside this corotation radius, they will be retrograde.

→ corotation; → radius.

That condition of a → galactic disk at an orbital radius in which the → angular velocity of the disk equals the → pattern speed. It is significant that the spiral wave pattern rotates as a rigid body (Ω_P = const), whereas the galactic disk rotates differentially (Ω is a function of galactocentric distance r). The distance r_C at which the two angular velocities coincide (Ω(r_C) = Ω_P) is referred to as the → corotation radius. The corotation resonance and its position within the galaxy is one of the fundamental properties of a spiral galaxy.

→ corotation; → resonance.