word-forming element used freely in English, "between, among, during," from Latin inter (prep., adv.) "among, between, betwixt, in the midst of" (also used extensively as a prefix), from PIE *enter "between, among" (source also of Sanskrit antar, Old Persian antar "among, between," Greek entera (plural) "intestines," Old Irish eter, Old Welsh ithr "among, between," Gothic undar, Old English under "under"), a comparative of root *en "in."
A living prefix in English from 15c. and used with Germanic as well as Latinate words. Spelled entre- in French; most words borrowed into English in that form were re-spelled 16c. to conform with Latin except entertain, enterprise. In Latin, spelling shifted to intel- before -l-, hence intelligence, etc.
late 14c., communioun, "participation in something; that which is common to all; union in religious worship, doctrine, or discipline," from Old French comunion "community, communion" (12c.), from Latin communionem (nominative communio) "fellowship, mutual participation, a sharing," used in Late Latin ecclesiastical language for "participation in the sacrament," from communis "common, general" (see common (adj.)).
Used by Augustine, in belief that the word was derived from com- "with, together" + unus "oneness, union." In English, from mid-15c. as "the sacrament of the Eucharist," from c. 1500 as "act of partaking in the sacrament of the Eucharist." From 1610s as "intercourse between two or more."
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Definitions of intercommunion from WordNet
participation in Holy Communion by members of more than one church (eg Catholic and Orthodox);