As generic terms, Indian Territory, The Indian Territories, or Indian Country are used to describe an evolving land area set aside by the United States Government for the relocation of the Indigenous peoples of the Americas who held Aboriginal title to their land. In general, the tribes ceded land they occupied in exchange for Land Grants in an area purchased by the US Federal Government from Napoleon, the Louisiana Purchase. The concept of an Indian Territory was an outcome of the 18th and 19th century policy of Indian removal. After the United States Civil War the policy of the government was one of assimilation. By 1907, when Oklahoma was admitted as a state, Indian Territory ceased to exist.
 
The term Indian Reserve is used to describe lands the British government set aside for Indigenous tribes between the Appalachian Mountains and the Mississippi River in the time before the Revolutionary War.
 
More specifically, Indian Territory was an Unorganized territory whose general borders were initially set by the Indian Intercourse Act of 1834, and was the successor to Missouri Territory after Missouri received statehood. The borders of Indian Territory were systematically reduced in size as various Organic Acts were passed by the US Congress creating an Incorporated territory or Territory of the United States that would eventually be admitted to the union as a State of the United States.
 
Several tribes, (Seneca, Osage and Pottawatomi, etc.) were relocated more than once as Indian Territory shrank in size. As tribes were relocated, some received land grants in exchange for their former lands, and others (such as Osage, Seminole, and Chickasaw) ultimately purchased their land, usually receiving Fee simple title to the land.
 
The Oklahoma Enabling Act specified that Oklahoma Territory and Indian Territory would be admitted as a single state of Oklahoma, which occurred in 1907. At that time, Indian Territory ceased to exist.

Definition of "Indian Territory"
Indian Territory, also known as the Indian Territories and the Indian Country, was land within the United States of America that was reserved for the forced re-settlement of Native Americans. The general borders were set by the Indian Intercourse Act of 1834.
 
While Congress passed several Organic Acts that provided a path for statehood for much of the original Indian Country, Congress never passed an Organic Act for the Indian Territory. Indian Territory was never an organized incorporated territory of the United States. In general, tribes could not sell land to non-Indians (Johnson v. M'Intosh). Treaties with the tribes severely restricted entry of non-Indians into tribal areas; Indian tribes were largely self-governing, were suzerain nations, with established tribal governments and well established cultures. The region never had a formal government until after the American Civil War. Therefore, the geographical location commonly called "Indian Territory" was not a traditional territory.
 
After the Civil War the Southern Treaty Commission re-wrote treaties with tribes that sided with the Confederacy, reducing the territory of the Five Civilized Tribes and providing land to resettle Plains Indians and tribes of the mid-west. These re-written treaties included provisions for a Territorial Legislature (which could only meet 30 days per year) with proportional representation from various tribes.
 
The Oklahoma organic act of 1890 created an organized incorporated territory of the United States of Oklahoma Territory, with the intent of combining the Oklahoma and Indian territories into a single State of Oklahoma. The residents of Indian Territory proposed to Congress that Indian Territory be admitted to the Union as the State of Sequoyah. However, Congress rejected the idea and Oklahoma became a state in 1907.

The citizens of Indian Territory tried, in 1905, to gain admission to the union as the State of Sequoyah, but were rebuffed by Congress and an Administration which did not want two new Western states, Sequoyah and Oklahoma. Citizens then joined to seek admission of a single state to the Union. With Oklahoma statehood in November 1907, Indian Territory was extinguished.
 
Many Indians continue to live in Oklahoma, especially in the eastern part.