1. An accumulation of dust and gas into larger bodies.
2. A process in which many smaller bodies congregate together under the action of forces to produce much larger bodies.

Such a force could be, for example, gravitation.

The ring of hot gas and dust that encircles a black hole, formed from material torn off the companion star to the black hole.

In losing its initial angular momentum, the hot material descends into the black hole, friction causing it to reach temperatures high enough to produce x-rays.

It is by the accretion ring emission that makes it possible for the presence of the black hole to be inferred.