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A spiral density wave is a spiral-shaped band of higher (or lower) density centered on the center of a disk (a flat region of many particles orbiting something massive, such as a circumstellar disk). The forces in such disks, which include gravity (i.e., self-gravity: of the disk material itself as well as that of larger objects embedded in the disk), radiation pressure, and MHD-forces, can be such that the disk does not stably remain uniform. It can develop patterns, such as spirals, termed spiral density waves.
The spiral density wave concept/mechanism (density wave model) has been used to explain the overall spiral structure of spiral galaxies, to explain such density waves observed in one of Saturns rings, and it occurs in simulations and analysis of disks such as protoplanetary disks and accretion disks.