The Arbiter-class starship is a Protoss warship specializing in providing psychic support.
Arbiters were crewed exclusively by Judicators, unlike other warships that were manned predominantly by Templar. The Judicator used the Arbiter as a base to provide support using space-time manipulation. Now, most Arbiters are crewed by Savrai Councillors.
The pilot of the Arbiter can generate a large reality-warping field that serves to conceal nearby friendly units. The Arbiter has to remain firmly anchored in space-time to generate the field and so is not concealed. Arbiters can also weaken space-time, tearing rifts in the fabric of space-time, creating a vortex linking another location to the Arbiter's location. This can be used to move personnel and supplies over long distances. On the other hand Arbiters can also strengthen space-time and create discrete pockets of space-time, trapping objects and preventing those objects from interacting with other objects outside of their pockets.
After the Judicator Caste was eliminated from the Protoss power structure, the Arbiter quickly fell into disuse. When the Protoss fled Aiur to Shakuras, only the High Arbiters of warbands such as Dabiri's Shadow company remained. These were quickly re-implimented by the Rhel'Kaeli Tribe, and soon by other tribes.