early 15c., "action of joining one thing to another," also "agreement, accord," also "state of matrimony," from Anglo-French unioun, Old French union (12c.), from Late Latin unionem (nominative unio) "oneness, unity, a uniting," also in Latin meaning "a single pearl or onion," from unus "one," from PIE root *oi-no- "one, unique."

Sense of "action of uniting into one political body" is attested from 1540s. Meaning "group of people or states" is from 1650s. Short for trade union, it is recorded from 1833. U.S. political sense is attested from 1775; used especially during the Civil War, in reference to the remainder of the United States after the Southern secession.

1670s, from union + jack (n.); properly a small British union flag flown as the jack of a ship, but it has long been in use as a general name for the union flag. The Union flag (1630s) was introduced to symbolize the union of the crowns of England and Scotland (in 1603) and was formed of a combination of the cross saltire of St. Andrew and the cross of St. George. The cross saltire of St. Patrick was added 1801 upon the union of parliaments of Great Britain and Ireland.

1841, "make into a union" (transitive), from union + -ize. Sense "form into a trade union" is from 1887. Related: Unionized; unionizing.

late 15c., "severance of union, disjunction," from dis- + union. Meaning "a breach of amity, contentious disagreement" is from c. 1600. In U.S. history, disunionist (1831) was "one who favors or seeks the disunion of the United States."

by 2000, the usual U.S. term for legally recognized same-sex unions short of marriage.

informal name of the Union of Soviet Socialist Republics; in use in U.S. newspapers by October 1919.

c. 1600, "act of coming together again," from re- "back, again" + union; or from French réunion (1540s). Meaning "a meeting of persons of previous connection" is from 1820.

The island of Reunion, formerly known as Bourbon, was renamed during the French Revolution (1793) in commemoration of the 1792 union of revolutionaries from Marseilles with the National Guard in Paris, renamed back to Bourbon after 1815, then back to the Revolutionary name after 1848.

Proto-Indo-European root meaning "one, unique."

It forms all or part of: a (1) indefinite article; alone; an; Angus; anon; atone; any; eleven; inch (n.1) "linear measure, one-twelfth of a foot;" lone; lonely; non-; none; null; once; one; ounce (n.1) unit of weight; quincunx; triune; unanimous; unary; une; uni-; Uniate; unilateral; uncial; unicorn; union; unique; unison; unite; unity; universal; universe; university; zollverein.

It is the hypothetical source of/evidence for its existence is provided by: Greek oinos "ace (on dice);" Latin unus "one;" Old Persian aivam; Old Church Slavonic -inu, ino-; Lithuanian vienas; Old Irish oin; Breton un "one;" Old English an, German ein, Gothic ains "one."

union of bones, 1570s, medical Latin, from Greek symphysis "a growing together, union," from assimilated form of assimilated form of syn "together" (see syn-) + physis "growth" (from PIE root *bheue- "to be, exist, grow"). Related: Symphytic.

1843, from German Zollverein, literally "customs union," from Zoll "toll" (see toll (n.)) + Verein "union," from vereinen "to unite," from ver- + ein "one" (from PIE root *oi-no- "one, unique").