An accretion disk is a formation that is created when matter falls into a gravitational source, such as a neutron star or wormhole.
In 2332, the USS Enterprise-C observed the accretion disk that had formed around a pulsar which was extracting material from a neighboring subgiant star. (ST short story: "Hour of Fire")
In 2374, when entering a subspace compression anomaly, the USS Rubicon was shrunk upon entering the accretion disk of the anomaly. (DS9 episode: "One Little Ship")