From the ArcaMax Publishing, Vocabulary Newsletter:
http://www.arcamax.com/news/vocabulary/s-565207-774627
arbiter \AR-buh-tuhr\ (noun) - 1 : A person appointed or chosen to
judge or decide a dispute. 2 : Any person who has the power of judging
and determining.
"If he was superior to the dog, the arbiter of its fate, wasn't God in
like measure the arbiter of human fate?" -- Czesław Miłosz, 'The
Issa Valley'
Arbiter is from Latin arbiter, "a witness, a spectator," hence "a
judge of any matter."