An arbiter is someone with the authority to make final decisions in a dispute or to judge what is correct, acceptable, or high-quality. When two people disagree about whether a baseball was fair or foul, the umpire acts as the arbiter, making a judgment that both sides must accept. In everyday life, parents often serve as arbiters when siblings argue about whose turn it is or who started the fight.

The word comes from the idea of someone standing above a conflict with the power to settle it. A referee is an arbiter of whether a goal counts. A teacher might be the arbiter of whether your science project meets the requirements. Some people become arbiters of taste: a famous food critic might be considered an arbiter of which restaurants are truly excellent, meaning their opinion carries special weight.

Notice that being an arbiter requires both authority and trust. You can't just declare yourself the arbiter of an argument between friends unless they agree to let you decide. The power comes from others accepting your judgment. When someone is described as “the final arbiter” of something, it means their decision ends the discussion.