Arbiter
Etymology
From Latin "arbiter" (a judge, an eyewitness who could rule on what he saw). Ancient Rome prized the "arbiter elegantiae," the acknowledged authority on style and taste. English kept both senses: the one who settles conflicts and the one who dictates fashion.
noun · /AR-bih-tur/
A person with the power or authority to settle a dispute or judge what is correct.
What it names
The final word — whoever gets to decide, whether in a quarrel or in matters of taste.
Example
Over the years she became the office’s unofficial arbiter of every scheduling dispute.
Related idea
An arbiter has broad, sometimes informal authority to decide; an arbitrator is the narrower legal role — a neutral third party formally appointed to resolve a specific case.