An Arbiter was a member of the judiciary on Earth, roughly equivalent to a judge in archaic legal systems.
Roj Blake's trial was overseen by an arbiter and two other members (no titles given) of a tribunal, although actual judgement and apparently sentencing as well was carried out by computer. Arbiter appears to be the formal title of a judicial authority within the Administration. (A: The Way Back)
Samor described himself as a pro tem, military arbiter.