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, indicating black-body radiation for something large and cooler than the star. When the disk can be resolved, the material making up the disk and how it varies by radius can be determined from this electromagnetic radiation, which is affected by the dust grain size: the emission of grains is inefficient for wavelengths long in comparison to the grain's diameter.
Older stars can also have such disks, such as debris disks.