An accretion disk was a structure that was created when matter falls into a gravitational source, like a wormhole.
Barzan scientists theorized that radiation buildup in the Barzan wormhole's accretion disk was responsible for its brief visible burst every 233 minutes. (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 and her crew to experience miniaturization. (DS9: "One Little Ship")