An accretion disk is a structure that is created when matter falls into a gravitational source, like a wormhole.
One of the theories 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 disc was also present in a subspace compression studied by the USS Defiant and the USS Rubicon in 2374. Entering this accretion disc caused the Danube class runabout to shrink. (DS9: "One Little Ship")