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The term ultra diffuse galaxy (UDG) was coined for galaxies with extremely low luminosity with the 2015 discovery of some examples in the Coma Cluster, and the term has since been used for similar galaxies found elsewhere. They lack the gas to feed star formation and tend to have older stars, and if star formation has become miniscule, a greater percentage of its stars are late-type, with low luminosity. Analogous galaxies have been reported back to at least 1984, using other terms with similar criteria, e.g., low-luminosity dwarfs, low-surface-brightness galaxies, etc.