If management and union officials cannot resolve a grievance, a(n) is asked to listen to the arguments of each side and to make a decision that both sides will have to comply with.

Arbitrator

Explanation: In simple words, arbitrator refers to an individual who is officially appointed by the court to settle dispute between two parties. Both the parties have to agree to the decision made by the arbitrator and it is legally enforceable in the court as well.  

Arbitration is a mechanism during which a conflict is settled by a neutral arbitrator whose judgment has been decided or declared by the respondents to the conflict will be definitive and binding. Testing and challenges of arbitration judgments are minimal.

Arbitration is regularly used in customer and employ ability issues in some nations such as United States, in which arbitration may be required by employment terms or legal agreements which can include an exemption of the chance to bring an allegation.