What is the name of the bright ring of hot plasma surrounding some black holes?

An accretion disk is the bright ring or flattened flow of hot plasma surrounding some black holes.

Gas and dust falling toward a black hole usually possess angular momentum, so they spiral inward instead of dropping straight down. Collisions, turbulence, and magnetic effects convert orbital energy into heat. The resulting disk can radiate strongly across the electromagnetic spectrum, especially in ultraviolet and X-ray light.

The disk is not the black hole itself. It is material outside the event horizon, and its brightness depends on the amount of matter supplied and the efficiency of the accretion process. Inner regions orbit extremely rapidly and are affected by strong gravity and relativistic effects.

Accretion disks also occur around young stars, white dwarfs, and neutron stars. Around black holes, however, the disk’s inner edge and emitted radiation can reveal information about the black hole’s mass and spin. Some systems additionally launch jets perpendicular to the disk.