A type of galaxy whose main structure is a thin disk of stars in approximately circular orbits about the centre and whose light output typically falls off exponentially with radius. The term applies to all types of galaxy other than ellipticals, dwarf spheroidals, and some peculiar galaxies. The disk in lenticular galaxies (type S0) contains very little interstellar material, while the disks of spiral and irregular galaxies contain considerable amounts of gas and dust in addition to the stars.