arbiter

Short definition from Wiktionary

Noun

1. 1
   A person appointed, or chosen, by parties to determine a controversy between them; an arbitrator.
2. 2
   A person or object having the power of judging, determining, or ordaining; one whose power of deciding and governing is not limited.
3. 3
   A component in circuitry that allocates scarce resources.

Verb

1. 1
   To act as an arbiter.