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."