Etymology
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mediator (n.)

mid-14c., mediatour, "one who intervenes between two parties (especially to seek to effect a reconciliation)," from Late Latin mediator "one who mediates," agent noun from stem of mediare "to intervene, mediate," also "to be or divide in the middle," from Latin medius "middle" (from PIE root *medhyo- "middle"). Originally applied to Christ, who in Christian theology mediates between God and man. Meaning "one who intervenes between two disputing parties for the purpose of effecting reconciliation" is first attested late 14c. Feminine form mediatrix (originally of the Virgin Mary) from c. 1400. Related: Mediatorial; mediatory.

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mediate (v.)

1540s, "divide in two equal parts" (a sense now obsolete), from Latin mediatus, past participle of mediare "to halve," later, "be in the middle," from Latin medius "middle" (from PIE root *medhyo- "middle"); from 1640s as "occupy a middle place or position." Meaning "act as a mediator, intervene for the purpose of reconciliation" is from 1610s; that of "settle by mediation, harmonize, reconcile" is from 1560s, perhaps back-formations from mediation or mediator. Related: Mediated; mediates; mediating.

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arbitrer (n.)
"arbitrator," late 14c., from Anglo-French arbitrour, Old French arbitreor "arbitrator, judge" (13c.), from Old French arbitrer "a judge, umpire, mediator" (see arbiter). Fem. form arbitress is from mid-14c.
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intercessor (n.)
"one who pleads or intervenes on behalf of another," late 15c., from a specific Christian use of Latin intercessor "one who intervenes, a mediator," agent noun from intercedere (see intercede). Related: Intercessory.
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go-between (n.)
"one who passes between parties in a negotiation or intrigue," 1590s, from verbal phrase go between in obsolete sense "act as a mediator" (1540s), from go (v.) + between.
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interneuron (n.)
1939, from neuron + first element from internuncial (adj.) "communicating between different parts of the body," from Latin internuncius "a messenger, mediator," from inter "between" (see inter-) + nuntius "messenger" (from PIE root *neu- "to shout").
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intermediary (adj.)

1757, "situated between two things;" 1818 as "serving as a mediator;" from French intermédiaire (17c.), from Latin intermedius "that which is between" (see intermediate). As a noun, "one who acts between others" from 1791 (Medieval Latin intermedium also was used as a noun). An earlier adjective was intermedial (1590s).

Intermediary, n., is, even with concrete sense of go-between or middleman or mediator, a word that should be viewed with suspicion & resorted to only when it is clear that every more ordinary word comes short of the need. [Fowler]
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reformer (n.)

mid-15c., "corrector, improver; mediator, negotiator," agent noun from reform (v.). From 1540s as "one who leads or assists the religious movements of the 16c. aimed at reformation of Christianity;" also "one who promotes or favors reform in certain practices of things."

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arbitrage (n.)
"arbitration, exercise of the function of an arbitrator," late 15c., from Old French arbitrage "arbitration, judgment," from arbitrer "to arbitrate, judge," from Late Latin arbitrari, from Latin arbiter "judge, umpire, mediator" (see arbiter). In finance, "the business founded on a calculation of the temporary differences in the price of securities in different markets" (1875).
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mediation (n.)

late 14c., mediacioun, "intervention, agency or action as a mediator or intermediary," from Medieval Latin mediationem (nominative mediatio) "a division in the middle," noun of action from past-participle stem of Latin mediare "to halve; to be in the middle," from medius "middle" (from PIE root *medhyo- "middle"). Related: Mediational.

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