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.
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.
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."