late Old English, "act of marching or proceeding," from Old French procession "procession" (religious or secular), 11c., and directly from Late Latin processionem (nominative processio) "religious procession," in classical Latin "a marching onward, a going forward, advance," noun of action from past participle stem of procedere (see proceed).
in Christianity, organized body of people advancing in formal or ceremonial manner as an element of Christian ritual or as a less official expression of popular piety. Public processions seem to have come into vogue soon after the recognition of Christianity as the religion of the Roman Empire by Constantine in the 4th century.