A more thorough explanation:

An Act of Union is a term used to describe several acts of Parliament that united different parts of Great Britain. The term can refer to:

• The Laws in Wales Act (1535), which united Wales with England and made it subject to English law.
• The Union with Ireland Act (1800), which abolished the Irish Parliament and incorporated Ireland into the United Kingdom of Great Britain and Ireland.
• The Union with Scotland in 1707, which was made by a treaty approved by separate acts of the parliaments of Scotland and England. This treaty dissolved each parliament and created the new state of Great Britain with one parliament, the Parliament of Great Britain.

For example, the Laws in Wales Act of 1535 united Wales with England and made it subject to English law. This meant that Wales was no longer a separate principality with its own laws and customs.

The Act of Union with Ireland in 1800 abolished the Irish Parliament and incorporated Ireland into the United Kingdom of Great Britain and Ireland. This meant that Ireland was no longer a separate country with its own government and laws.

The Union with Scotland in 1707 was made by a treaty between Scotland and England. This treaty dissolved each parliament and created the new state of Great Britain with one parliament, the Parliament of Great Britain. This meant that Scotland was no longer a separate country with its own parliament and laws.