union has 21 distinct synonyms spread across 9 of its 13 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its 3 recorded opposites are separation, disunion, nonunion.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
an organization of employees formed to bargain with the employer
“you have to join the union in order to get a job”
the United States (especially the northern states during the American Civil War)
“he has visited every state in the Union”
the act of pairing a male and female for reproductive purposes
“the casual couplings of adolescents”
the state of being joined or united or linked
“there is strength in union”
the state of being a married couple voluntarily joined for life (or until divorce)
“a long and happy marriage”
healing process involving the growing together of the edges of a wound or the growing together of broken bones
a political unit formed from previously independent people or organizations
“the Soviet Union”
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”
the occurrence of a uniting of separate parts
“lightning produced an unusual union of the metals”
a device on a national flag emblematic of the union of two or more sovereignties (typically in the upper inner corner)
the act of making or becoming a single unit
“the union of opposing factions”
being of or having to do with the northern United States and those loyal to the Union during the American Civil War
“Union soldiers”
Synonyms, antonyms and the broader and narrower relations for union come from Princeton University’s WordNet. Full sources and licences.