1With following numeral: any of certain galaxy clusters distinguished by their richness (i.e. the number and density of galaxies in their central regions) and listed by Abell in his catalogue.
2Attributive and in the possessive. Designating rich galaxy clusters listed in Abell's catalogue; relating to such clusters. Frequently in "Abell cluster".
1950s; earliest use found in Astrophysical Journal. From the name of George Ogden Abell, U.S. astronomer, who compiled a catalogue of clusters of galaxies, published in 1958.