The Dagellan Cluster was a star cluster.
When Jason Kane was there, it was simply known as 'the Cluster'. By Bernice Summerfield's time it was known as the Dagellan Cluster. She told Jason she believed it had been named after the captain of the first Earth ship to reach it.
The Cluster was the home to a loose confederation of about four hundred non-aligned worlds. These surrounded three competing empires, the Czhan, Dakhaari and Saloi. The Skrak were manipulating those empires as part of a plan to take over the universe. They were stopped by the Seventh Doctor.
The star systems of the Dagellan Cluster were connected by a three-dimensional latticework of standing electro gravitational fields. Spaceships could slowly travel along them via a relatively low-energy and low maintenance process. The standing fields did not extend beyond the Cluster, making intergalactic transport more difficult. (PROSE: Death and Diplomacy)