Act of Union can mean:

United Kingdom

The Act of Union is a name given to several acts passed by the English, Scottish and British Parliaments from 1536 onwards. Among those:

• Acts of Union 1536-1543 joining England and Wales (as the Kingdom of England)
• Act of Union 1707 joining the Kingdoms of Scotland and England (to form the Kingdom of Great Britain)
• Act of Union 1800 joining Ireland to Great Britain (to form the United Kingdom of Great Britain and Ireland)
• Act of Union 1840 joining Upper Canada and Lower Canada (to form the Province of Canada).

Sweden

• Act of Union 1814 forming a personal union between Sweden and Norway.

Other meanings

The term "act of union" is also used as a euphemism for sexual intercourse.