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The term proto-cluster (or protocluster) is used for galaxy clusters at very high redshift, such as galaxies grouped together in an era in which relatively few galaxies are known, an example being SSA22. It is presumed that they are young and the term may be used for galaxy clusters judged to be young for other reasons, or for a region with signs that a galaxy cluster is forming. The term proto-cluster candidate is common since observational evidence is often initially inconclusive.
The term proto-cluster has also been used for very young stellar clusters, such as IRAS 22134+5834, i.e., proto stellar clusters rather than proto galaxy clusters,