ABSTRACT
Galaxies are immense, gravitationally bound collections of stars, dust, gas, and dark matter. They tend to be found close together, in groups and clusters, with interactions and mergers rather common and important in their overall development. Clusters of galaxies show evidence of a connection between the environment and the evolution of galaxies, and by looking at large redshifts it is now possible to observe the evolution of the whole galaxy population with cosmic time. This entry provides definitions and details the properties and components associated with galaxies. Clusters of galaxies and the evolution of galaxies are also detailed.
