arbiter
noun [ C ]
uk/ˈɑːbɪtər/ ussomeone whose opinion has a lot of influence:
arbiter of fashion/style/taste He rose from polite but humble beginnings to become an arbiter of taste and manners.
LAW
a person or organization that has the power to make a decision about what is acceptable or legal:
the final/sole/ultimate arbiter It is the country's parliament which remains the final arbiter of which laws are passed and in what form.