The innermost stable circular orbit around a black hole is commonly abbreviated as ISCO.
The ISCO is the smallest orbit in which matter can circle a black hole indefinitely without losing stability. Inside it, material generally plunges inward rather than remaining in a circular path. Its radius depends on the black hole’s mass and spin.
For a nonrotating black hole, the ISCO lies at three Schwarzschild radii from the centre. A rapidly rotating black hole can have a smaller prograde ISCO because frame dragging changes the orbital structure. The orbit is therefore important in models of accretion disks and X-ray emission.
ISCO is not the same as the event horizon or the photon sphere. The event horizon is a one-way boundary, while the ISCO concerns the stability of matter orbiting outside that boundary. Astronomers use estimates of the inner disk edge to infer black-hole spin.