A group of galaxies bound together by gravity.

A group of galaxies held together by their mutual gravitational attraction (and not to be confused with the term galactic cluster).

a gravitationally bound assemblage of dozens to thousands of galaxies

A collection of galaxies that is drawn together by gravity. Galaxy clusters are the largest objects in the universe that have a gravitational pull strong enough to overcome the Hubble Expansion, and thus galaxies in a cluster to not expand away from each other.

A system of galaxies containing from a few to a few thousand member galaxies that are all gravitationally bound to each other.

Galaxies can swarm together to form groups and clusters of galaxies held together by their mutual gravity. X-ray observations show that these enormous systems of galaxies are filled with colossal clouds of hot gas. These clouds have temperatures as high as a hundred million degrees and contain as much mass as all the stars in the galaxies in the cluster. [More Info: Field Guide

A collection of dozens to thousands of galaxies bound together by gravity.