The Treaty of Union is the name usually now given to the treaty which led to the creation of the new political state of Great Britain. The treaty, effective since 1707, brought the Kingdom of England (which already included Wales) and the Kingdom of Scotland together to be "United into One Kingdom by the Name of Great Britain". At the time it was more often referred to as the Articles of Union.
The Treaty details were agreed on 22 July 1706, and separate Acts of Union were then passed by the parliaments of England and Scotland to put the agreed Articles into effect. The Treaty of Union was eventually passed in the Parliament of Scotland.