In the Halo universe, the Arbiter is a ceremonial, religious, and political rank bestowed upon alien Covenant Elites.
Thel 'Vadam (previously known as Thel 'Vadamee, the "-ee" suffix was dropped after the Great Schism) is the most prominent Sangheili and Arbiter in the games. https://en.wikipedia.org/wiki/Arbiter_(Halo)
https://halo.fandom.com/wiki/Thel_'Vadam