Union generally means that two or more things are joined into one. That can be an organization of several people or organizations, several objects combined into one, and so on. The term may mean:

In politics
 The Union, referring to the federation that is the United States
 The Union (American Civil War), the states that did not declare their secession
 The Union Army, the army of the United States in the Civil War
 National Union Party (United States), forerunner of the United States Republican Party during the 1864 election
 Act of Union, one of several acts by the English Parliament
 United Kingdom, the constitutional result of that legislation
 African Union, an inter-governmental organization in Africa
 European Union, a supranational and inter-governmental organization in Europe
 Government of India, known officially as the Union Government
 Personal union, a group of separate, independent states that share the same monarch
 Political union, a type of state composed of smaller states
 Real union, an intermediate step between personal and political union of states
 Soviet Union, Union of Soviet Socialist Republics
 State of the Union Address annual speech by the President of the United States
 The Union (political coalition) (l'Unione), a twenty-first century Italian political coalition
 Unionist Party (Canada), known when in power as the Union Government
 Trade union, labor union, employment

In sports and entertainment
 Unión de Santa Fe, Argentine professional football club
 R. Union Saint-Gilloise, Belgian professional football club
 Club Atlético Unión de Sunchales, lower league Argentine football club
 Unión Magdalena a Colombian professional football club
 The Union, a group of professional wrestlers in opposition to Vince McMahon in the World Wrestling Federation
 The supporters' club of the A-League football side Melbourne Victory
 Rugby Union
 Union (album), a 1991 album by the musical group Yes
 Union (band), an American heavy metal band
 Union (comic), set in the Star Wars universe
 Union (comics), a character published by Wildstorm
 "Union", a song by Avail from their 2000 album One Wrench
 "Union", a song by the Black Eyed Peas from their album Monkey Business

In education
The term student union can be used in either of two senses:
 Students' union, a student organization at many colleges and universities dedicated to student governance (not common in USA, see below)
 Student activity center, a building that typically hosts a "union" of student services (e.g., food service, health facilities, lounges) at such institutions (also known as a "student center" or "student commons")

In the United States, the phrase more commonly refers to the building. The phrase "student government" or "student senate" is usually used when referring to the subject of the "Students' union" article.
 Union College, a college for which the principal campus sits in Schenectady, New York
 Union College (Kentucky), a college in Kentucky
 Union Institute & University, a non-traditional educational institution
 Union Public Schools, a public school system in Tulsa, Oklahoma
 Union Theological College, a college in Northern Ireland
 Union Theological Seminary in the City of New York, an ecumenical Christian seminary in New York City, New York
 Union Theological Seminary & Presbyterian School of Christian Education, a Christian seminary in Richmond, Virginia
 Union University, a Baptist college in Tennessee

In transportation
 Union Railroad, any of several railroad companies
 Union (ship), a ship sunk in Fiji in 1804

In tradesmanship
 Union (electrical), a term sometimes used for a butt connector used to splice wires
 Union (hardware), a term sometimes used for a standoff that serves as an elongated nut
 Union (plumbing), a pipe fitting

In numismatics
 Union (coinage), a US gold coin that was worth $100 face value, it was never made because it would only be circulated among banks.

In mathematics and computer science
 Union (set theory)
 Union (SQL), an operator that combines the result of two SQL SELECT queries into a single result table
 Union (computer science), a data structure that stores several types of data in the same location.
 Union mount, a way to access separate file system directories as if they were but one.

As a surname
 Gabrielle Union, an American actress