What is the 12th Amendment of the Indian Constitution, in simple terms?

The Twelfth Amendment Act, 1962 incorporated Goa, Daman and Diu in the Indian Union. India acquired the three territories such as Goa, Daman and Diu from the Portuguese by means of police action in 1961. They were constituted as a union territory by the 12th Constitutional Amendment Act, 1962. Later, in 1987, Goa has conferred a statehood. Consequently, Daman and Diu were made a separate union territory. 

Further Reading :

• Important Amendments in Indian Constitution
• Types of Amendments in the Indian Constitution