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The term innermost stable circular orbit (ISCO) is used for black holes and neutron stars: an orbit at the distance from the center of the object (the ISCO-radius) immediately beyond that for which general-relativity effects make such an orbit unstable. The rotation of the host object affects this distance, but for the simple case of no rotation:
RISCO = 6GM/c² = 3RS
The volume between the ISCO-radius and the event horizon is termed the plunging region, where matter generally falls into the black hole.