The common abbreviation for the innermost stable circular orbit around a black hole is ISCO.
The ISCO is the smallest radius at which a test particle can maintain a stable circular orbit in general relativity. Move inward past that boundary, and even a tiny disturbance can make the particle spiral inward rather than settle into another stable circular path. This makes the ISCO especially important for modeling accretion disks.
For a non-rotating Schwarzschild black hole, the ISCO lies at 6GM/c², or three Schwarzschild radii. A spinning Kerr black hole changes the result: an orbit moving in the same direction as the black hole’s spin can lie closer in, while a counter-rotating orbit lies farther out.
The ISCO is not the event horizon, photon sphere, or Roche limit. It describes orbital stability for an idealized test particle. Astronomers use the predicted ISCO location to interpret the inner edge of accretion disks, estimate black-hole spin, and model the motion of matter and compact objects in strong gravity.