un·ion / ˈyoōnyən/ • n. 1. the action or fact of joining or being joined, esp. in a political context: he was opposed to closer political or economic union with Europe | a currency union between the two countries. ∎  a state of harmony or agreement: they live in perfect union. ∎  a marriage: their union had not been blessed with children. 2. an organized association of workers formed to protect and further their rights and interests; a labor union: the National Farmers' Union. ∎  a club, society, or association formed by people with a common interest or purpose: members of the Students' Union. ∎ short for rugby union. ∎ Brit., hist. a number of parishes consolidated for the purposes of administering the Poor Laws. ∎  (also un·ion work·house or un·ion house) a workhouse set up by such a group of parishes. ∎  Brit. an association of independent churches for purposes of cooperation. 3. (also Un·ion) a political unit consisting of a number of states or provinces with the same central government, in particular: ∎  the U.S., esp. from its founding by the original thirteen states in 1787–90 to the secession of the Confederate states in 1860–61. ∎  (also the Fed·er·al Un·ion) the northern states of the U.S. that opposed the seceding Confederate states in the Civil War. 4. a building at a college or university used by students for recreation and other nonacademic activities. 5. Math. the set that comprises all the elements (and no others) contained in any of two or more given sets. ∎  the operation of forming such a set. 6. a pipe coupling. 7. a part of a flag with an emblem symbolizing national union, typically occupying the upper corner next to the staff.