A system of galaxies containing several to thousands of member galaxies.
An aggregate of galaxies. Clusters may range in richness from loose groups, such as the Local Group, with 10 to 100 members, to great clusters of over 1000 galaxies.
A system of galaxies containing from a few to a few thousand member galaxies which are all gravitationally bound to each other.
A concentration of galaxies bound together by gravity. The term 'cluster' usually refers to a large collection of many tens or hundreds of galaxies and the term 'group' is used for a much smaller grouping.
A group of galaxies that are bound together by gravity.