An arbiter is a person (not a judge or magistrate) who mediates disputes out of court.
Els Renora served as an arbiter of the Klaestron's demand to extradite Jadzia Dax in 2369. (DS9: "Dax")
Odo intended to bring Quark in front of an arbiter for smuggling kemocite. (DS9: "Little Green Men")
Several months later, an arbiter on Deep Space 9 sentenced Ojuy Gel to three weeks of community service on sanitation duty. (DS9: "Crossfire")
In 2374, an enraged Dukat angrily called Benjamin Sisko "the supreme arbiter of right and wrong in the universe". (DS9: "Waltz")