An aggregate of galaxies. Bautz and Morgan divide them into three morfological types: type I contains a supergiant cD galaxy; type III contains no members significantly brighter than the general bright population. Coma is type II, Virgo is type III. Rood tecognizes three types: A compact group contains a few galaxies separated by a few galaxy diameters. A loose group contains on the order of 10 galaxies separated by 10-100 or more galaxies within a volume comparable to that of a loose group.