a person or agent that has the ultimate power to settle a particular dispute;
e.g. The court acted as arbiter in the conflicts between the members of the family.
(noun)
a person or agent whose actions of views are considered an authority on a particular matter and whose opinion or judgement therefore has significant influence;
e.g. She may be considered an arbiter on healthy eating, since she is a qualified dietitian and has written several books on the subject.