arbiter

sovittelija, välimies

välitystuomari A person appointed, or chosen, by parties to determine a controversy between them; an arbitrator.
1931, William Bennett Munro, The government of the United States, national, state, and local, page 495

In order to protect individual liberty there must be an arbiter between the governing powers and the governed.

Television and film, not Vogue'' and similar magazines, are the arbiters of fashion.

Worse, since there was no institution to arbiter disagreements between Parliament and the government, whenever Parliament voted against the government on the smallest issues, coalitions fragmented, and governments had to be recomposed.