the.com/union

means A union is the joining of two or more things, people, or groups into a single whole — whether that's a labor organization, a marriage, a coalition of states, or the overlap of two sets in math.

from From Latin 'unio,' meaning oneness — and curiously, the same word the Romans used for a single large pearl, the kind that came one to an oyster. 'Unio' grew straight from 'unus,' 'one,' the same root that seeded 'unit,' 'unite,' and 'universe.' It reached English through Old French 'union' in the late Middle Ages, first carrying the sense of harmony and joining-together; the political and labor meanings — trade unions, the United States as a 'Union' — are later flowerings of that single, ancient idea of many becoming one.