Accretion disk: A swirling, luminous disk of gas that surrounds a central object (usually a star) and from which matter falls onto the central object. Accretion disks are formed when material from the surrounding medium falls onto the central object, often due to the gravitational pull of the central object. The incoming material forms a disk-like structure as it orbits the central object, and as it falls inward it heats up due to friction and other interactions. This heating causes the disk to emit radiation, making it visible.