A Circumstellar Disk is matter, typically in a ring, collected around a star. It can be composed of gas, Dust, Planetesimals, or a combination, in orbit around the star. For young stars, for which the term Protoplanetary Disk is used, it may be the raw material of later planets. The Asteroids around the Sun represent a thin circumstellar disk.
Excess Infrared from a star is evidence of a disk.
Older stars can also have disks such as Debris Disks.
Protoplanetary Disk (PPD)
Surface Density (Σ)