late 14c., "person who has power of judging absolutely according to his own pleasure in a dispute or issue," from Old French arbitre "arbiter, judge" (13c.) and directly from Latin arbiter "one who goes somewhere (as witness or judge)," in classical Latin used of spectators and eye-witnesses; specifically in law, "he who hears and decides a case, a judge, umpire, mediator;" from ad "to" (see ad-) + baetere "to come, go," a word of unknown etymology.
The attestations suggest that baetō was the original form which sometimes became bētō, while -bītō was regular in non-initial syllables (especially in Plautus). Where bītō occurs independently (4x in Plautus), it must be a decompounded form. [de Vaan]
The specific sense of "one chosen by two disputing parties to decide the matter" is from 1540s. Compare arbitrator. The earliest form of the word attested in English is the fem. noun arbitress (mid-14c.) "a woman who settles disputes." Gaius Petronius Arbiter (circa 27-66 C.E.) was a friend of Nero, noted voluptuary, reputed author of the "Satyricon," and an authority on matters of taste and style (elegantiae arbiter, punning on the name).
late 14c., "faculty of making a choice or decision, judgment, discretion;" early 15c., "authority or responsibility for deciding a dispute," from Old French arbitracion and directly from Latin arbitrationem (nominative arbitratio) "judgment, will," noun of action from past-participle stem of arbitrari "to be of an opinion, give a decision," from arbiter "a judge, umpire, mediator" (see arbiter). Meaning "settlement of a dispute by a third party" is from 1630s. Related: Arbitrative.
The legal form of popular arbiter. In modern usage, an arbiter makes decisions of his own accord and is accountable to no one but himself; an arbitrator decides issues referred to him by the parties. "It is often the practice to appoint two or more arbitrators, with an umpire, chosen usually by them, as final referee" [OED].
"any mode of thought or life in which reliance is placed upon a spiritual illumination believed to transcend ordinary powers of understanding," 1736, from mystic (adj.) + -ism. Often especially in a religious sense, and since the Enlightenment a term of reproach, implying self-delusion or dreamy confusion of thought.
Mysticism and rationalism represent opposite poles of theology, rationalism regarding the reason as the highest faculty of man and the sole arbiter in all matters of religious doctrine; mysticism, on the other hand, declaring that spiritual truth cannot be apprehended by the logical faculty, nor adequately expressed in terms of the understanding. [Century Dictionary]