n. One who promotes or advocates union.n. A member of a trade-union; a trade-unionist. One who during the American civil war took the side of the national government.n. [capitalized] In British politics, one who is opposed to the dissolution or rupture of the legislative union existing between Great Britain and Ireland, and especially to the separatist principles and tendencies of those who desire to establish home rule in Ireland: a name applied to the Conservatives and Liberal Unionists.Of or pertaining to a union or to unionism; promoting or advocating union: as, a unionist movement; a unionist party.Specifically, during the civil war in the United States, of or pertaining to the Union party or cause.