The disk of stuff you see circling black holes in artist renderings like the one in the picture.
An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction causes orbiting material in the disk to spiral inward towards the central body.
Hear about accretion disks and a LOT more interesting and mind blowing stuff about astrophysics in the TEDRadio Hour podcast below!