A circumstellar disk (in context, just 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.