Pakistan (Bengali: পাকিস্তান অধিরাজ্য pakistan ôdhirajyô; Urdu: مملکتِ پاکستان‎ mumlikāt-ē pākistān), also called the Dominion of Pakistan, was an independent federal dominion in South Asia that was established in 1947 as a result of Independence of India, followed by the simultaneous Partition of India to create a new country called Pakistan. The dominion, which included much of modern-day Pakistan and Bangladesh, was conceived under the two-nation theory as an independent country composed of the muslim-majority areas of the former British India.

To begin with, it did not include the princely states of Pakistan, which acceded slowly between 1947 and 1948. In 1956 Pakistan was administratively split into the western wing named West Pakistan, and the province of East Bengal was renamed as the eastern wing named East Pakistan. In 1971 East Pakistan seceded from the union to become Bangladesh.

Section 1 of the Indian Independence Act 1947 provided that from "the fifteenth day of August, nineteen hundred and forty-seven, two independent dominions shall be set up in India, to be known respectively as India and Pakistan." India was treated by the United Nations as the successor state to the former British India. As it was already a member of the United Nations, India continued to hold its seat there and did not apply for a new membership. However, Pakistan was a newly created nation and needed to apply to join. It was admitted as a UN member on 30 September 1947, a few weeks after its independence The British monarch became head of state of the new dominion, with Pakistan sharing a king with the other Commonwealth realms, but the monarch's constitutional roles were delegated to the Governor-General of Pakistan, and most real powers resided with the new government headed by Jinnah.

...
Wikipedia