A star cluster was a group of stars.
A battle during the Rutan-Sontaran War was fought in the Madillon Cluster. (TV: The Two Doctors)
According to Commander Kaagh, half the species from Earth to the Verlixix Spiral Cluster had plans to invade Earth because humans had "no guts". (TV: Enemy of the Bane)
The Jolius Cluster was home to the Okrenians. (COMIC: Pay the Piper)