An accretion disk is a structure that is created when matter falls into a gravitational source, such as a black hole.

In Star Trek, 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. (TNG: "The Price"). An accretion disk was present in a subspace compression anomaly studied by the USS Defiant and the USS Rubicon in 2374. Entering this accretion disk caused the Danube-class runabout to shrink. (DS9: "One Little Ship")

In real science, an accretion disk is a indeed structure (often 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.