union (n.)

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.

Definitions of union
union (n.)
an organization of employees formed to bargain with the employer;
you have to join the union in order to get a job
Synonyms: labor union / trade union / trades union / brotherhood
union (n.)
the act of pairing a male and female for reproductive purposes;
Synonyms: coupling / mating / pairing / conjugation / sexual union
union (n.)
the state of being joined or united or linked;
there is strength in union
Synonyms: unification
union (n.)
the state of being a married couple voluntarily joined for life (or until divorce);
God bless this union
Synonyms: marriage / matrimony / spousal relationship / wedlock
union (n.)
healing process involving the growing together of the edges of a wound or the growing together of broken bones;
Synonyms: conglutination
union (n.)
a political unit formed from previously independent people or organizations;
union (n.)
a set containing all and only the members of two or more given sets;
let C be the union of the sets A and B
Synonyms: sum / join
union (n.)
the occurrence of a uniting of separate parts;
lightning produced an unusual union of the metals
union (n.)
a device on a national flag emblematic of the union of two or more sovereignties (typically in the upper inner corner);
union (n.)
the act of making or becoming a single unit;
the union of opposing factions
Synonyms: unification / uniting / conjugation / jointure
union (adj.)
of trade unions;
a union-shop clause in the contract
the union movement
union negotiations
Union (n.)
the United States (especially the northern states during the American Civil War);
Synonyms: North
Union (adj.)
being of or having to do with the northern United States and those loyal to the Union during the American Civil War;
Synonyms: Federal