An arbitrator according to the Oxford English Dictionary is a person to whom the conflicting parties agree to refer their claims to obtain an equitable decision. With respect I agree with Sir Edward Blackstone, but not with the Oxford English Dictionary. The dictionary is out of date. Today's arbitrator is controlled either at common law or, if the agreement to arbitrate is in writing, controlled by Statute or by particular Arbitral rules. I will come back to that alternative basis.
Arbitration antedates legal systems and courts.
In Roman times litigation was no more than private arbitration with the approval and assistance of a magistrate, the Praetor, elected annually for that purpose. Then, in the middle ages, English merchants resorted to arbitration to settle disputes long before the Kings Courts found any way to enforce contractual obligations. The English Parliament began to take a hand in 1698. Other Acts followed. The Arbitration Act 1889 provided for something called a 'Statement of Case'.