What is Arbiter?

Legal Definition
One who, decides without any control. A judge with the most extensive arbitrary powers; an arbitrator.
-- Bouviers Law Dictionary
Legal Definition
A person chosen to decide a controversy; an arbitrator, referee. A person bound to decide according to the rules of law and equity, as distinguished from an arbitrator, who may proceed wholly at his ov.n discretion, so that it be according to the judgment of a sound man. Cowell.

According to Mr. Abbott, the distinction is as follows: "Arbitrator" is a technical name of a person selected with reference to an established system for friendly determination of controversies, which, though not judicial, is yet regulated by law; so that the powers and duties of the arbitrator, when once he is chosen, are prescribed by law, and. his doings may be judicially revised if he has exceeded his authority. "Arbiter" is an untechnical designation of a person to whom a controversy is referred, irrespective of any law to govern the decision ; and is the proper word to signify a referee of a question, outside of or above municipal law. But it is elsewhere said that the distinction between arbiters and arbitrators is not observed in modem law. Russ. Arb. 112.

In the Roman law. A judge invested with a discretionary power. A person appointed by the praetor to examine and decide that class of causes or actions termed "bones fidei," and who had the power of judging according to the principles of equity, (ex aequo et bono;) distinguished from the judex, (q. v.,) who was bound to decide according to strict law. Inst 4, 6, 30, 31.
-- Black's Law Dictionary
Legal Definition
An arbitrator; formerly one who was governed by law and equity in his decisions rather than by his own judgment.
-- Ballentine's Law Dictionary