The Leave to Remain is the legal status of a person issued by a government office of internal affairs to one who is not yet a citizen. In most stable countries, Indefinite leave to remain (as is known in the UK) is granted to these foreign citizens after a specified period spent within its borders. This is usually the final step towards naturalization which in turn leads to full citizenship. Indefinite Leave to Remain or ILR, is an immigration status granted to a person who does not hold right of abode in the United Kingdom, but who has been admitted to the UK without any time limit on his stay and who is free to take up employment, without restriction. ... Naturalization is the act whereby a person voluntarily and actively acquires a nationality which is not his or her nationality at birth. ... Citizenship is membership in a political community (originally a city but now usually a country) and carries with it rights to political participation; a person having such membership is a citizen. ...

A Leave to Remain need not be indefinite.

Compare to Permanent Resident status in the United States and Landed Immigrant status in Canada.