c.1300, from L.L.
mediatorem (nom.
mediator) "one who mediates," from
mediatus, pp. of
mediari "to intervene, mediate," also "to be or divide in the middle," from L.
medius "middle" (see
medial). Originally applied to Christ, who in Christian theology "mediates" between God and man. Meaning "one who intervenes between two disputing parties" is first attested 1387. The verb
mediate is first recorded 1542, probably a back-formation from
mediation (c.1425) or
mediator.
[mee-dee-ey-ter] 
diˌeɪ