Same as arbitrator; the presiding and deciding officer(s) of an arbitration.
Scotland has a few twists in alternative dispute resolution terminology.
Preferring arbiter to arbitrator is one; using the term oversman (umpire) is another.
Some translation of Roman law documents, such as the Twelve Tables, defer to this spelling as well: arbiter. The Romans useed the word arbiter interchangeably with judex to refer to the office of judge in early Roman law.
Further Reading: