Someone with the power to settle matters at will:
— She was the final arbiter on all matters of fashion.

synonym: supreme authority.

Roget 694: director, manager, governor, rector, comptroller.    superintendent, supervisor, straw boss; intendant; overseer, overlooker^†; supercargo^†, husband, inspector, visitor, ranger, surveyor, aedile^†; moderator, monitor, taskmaster; master etc. 745; leader, ringleader, demagogue, corypheus, conductor, fugleman^†, precentor^†, bellwether, agitator; caporal^†, choregus^†, collector, file leader, flugelman^†, linkboy^†.    guiding star etc. (guidance) 693; adviser etc. 695; guide etc. (information) 527; pilot; helmsman; steersman, steermate^†; wire-puller.    driver, whip, Jehu, charioteer; coachman, carman, cabman; postilion, vetturino^†, muleteer, arriero^†, teamster; whipper in.    head, head man, head center, boss; principal, president, speaker; chair, chairman, chairwoman, chairperson; captain etc. (master) 745; superior; mayor etc. (civil authority) 745; vice president, prime minister, premier, vizier, grand vizier, eparch^†.    officer, functionary, minister, official, red-tapist^†, bureaucrat; man in office, Jack in office; office bearer; person in authority etc. 745.    statesman, strategist, legislator, lawgiver, politician, statist^†, statemonger^†; Minos, Draco; arbiter etc. (judge) 967; boss [U.S.], political dictator.    board etc. (council) 696.    secretary, secretary of state; Reis Effendi; vicar etc. (deputy) 759; steward, factor; agent etc. 758; bailiff, middleman; foreman, clerk of works; landreeve^†; factotum, major-domo^†, seneschal, housekeeper, shepherd, croupier; proctor, procurator.    ... show more

Roget 695: advice, counsel, adhortation^†; word to the wise; suggestion, submonition^†, recommendation, advocacy; advisement.    exhortation etc. (persuasion) 615; expostulation etc. (dissuasion) 616; admonition etc. (warning) 668; guidance etc. (direction) 693.    instruction, charge, injunction, obtestation^†; Governor's message, President's message; King's message, Queen's speech; message, speech from the throne.    adviser, prompter; counsel, counselor; monitor, mentor, Nestor, magnus Apollo [Lat.], senator; teacher etc. 540.    guide, manual, chart etc. (information) 527.    physician, doctor, leech^†, archiater^†.    arbiter etc. (judge) 967.    reference, referment^†; consultation, conference, pourparler.    ... show more

Someone chosen to judge and decide a disputed issue:
— The critic was considered to be an arbiter of modern literature.

synonyms: arbitrator, umpire.

Roget 967: judge; justice, justiciar^†, justiciary^†; chancellor; justice of assize, judge of assize; recorder, common sergeant; puisne judge, assistant judge, county court judge; conservator of the peace, justice of the peace; J.   P.; court etc. (tribunal) 966; magistrate, police magistrate, beak [Slang]; his worship, his honor, his lordship.    jury, twelve men in a box.    Lord Chancellor, Lord Justice; Master of the Rolls, Vice Chancellor; Lord Chief Justice, Chief Baron; Mr.    Justice, Associate Justice, Chief Justice; Baron, Baron of the Exchequer.    jurat [Lat.], assessor; arbiter, arbitrator; umpire; referee, referendary^†; revising barrister; domesman^†; censor etc. (critic) 480; barmaster^†, ephor^†; grand juror, grand juryman; juryman, talesman.    archon, tribune, praetor, syndic, podesta^†, mollah^†, ulema, mufti, cadi^†, kadi^†; Rhadamanthus^†.    litigant etc. (accusation) 938.    ... show more

Roget 480: result, conclusion, upshot; deduction, inference, ergotism [Med.]; illation; corollary, porism^†; moral.    estimation, valuation, appreciation, judication^†; dijudication^†, adjudication; arbitrament, arbitrement^†, arbitration; assessment, ponderation^†; valorization.    award, estimate; review, criticism, critique, notice, report.    decision, determination, judgment, finding, verdict, sentence, decree; findings of fact; findings of law; res judicata [Lat.].    plebiscite, voice, casting vote; vote etc. (choice) 609; opinion etc. (belief) 484; good judgment etc. (wisdom) 498.    judge, umpire; arbiter, arbitrator; asessor, referee.    censor, reviewer, critic; connoisseur; commentator etc. 524; inspector, inspecting officer.    twenty-twenty hindsight [judgment after the fact]; armchair general, monday morning quarterback.    ... show more

Dutch: bemiddelaar, scheidsrechter, arbiter
Polish: osądziciel