An accretion disk is a structure that is created when matter falls into a
gravitational source, such as a black hole. It is a circumstellar disk formed by diffuse material in orbital motion around a central body. The central body is typically either a young star, a protostar, a white dwarf, a neutron star, or a black hole. Instabilities within the disc redistribute angular momentum, causing material in the disc to spiral inward towards the central body. Gravitational energy released in that process is transformed into heat and emitted at the disk surface in the form of electromagnetic radiation.

In the Star Trek:The Next Generation episode "The Price", one of the theories as to why the Barzan wormhole remained stable for a time stated that a radiation buildup in the accretion disk was responsible for the phenomenon. An accretion disk was present in a subspace compression anomaly studied by the USS Defiant and the USS Rubicon in the Star Trek:Deep Space Nine episode "One Little Ship". Entering this accretion disk caused a Danube-class runabout to shrink.

Chandra image of an accretion disk in
Radio Galaxy 3C 390.3