A small, faint type of galaxy with a diameter between about 60 and 200 l.y. and containing some 10 million stars, midway in size between a globular cluster and a dwarf elliptical galaxy. Discovered in 2000 in the Fornax cluster, UCDs are thought to be the nuclei of dwarf elliptical galaxies whose outer layers have been stripped away by the tidal effects of much larger neighbour galaxies. Some ultra-luminous globular clusters around giant elliptical galaxies may in fact be UCDs.