It is a branch of law that covers U.S. citizenship, loss of citizenship, and the permission and removal of strangers. It refers to national government policies which control the phenomenon of immigration to their country.
Immigration law, concerning overseas citizens, is associated to nationality law, which governs the legal status of people, in subjects such as citizenship. This body of law varies from country to country, as well as according to the political environment of the times, as sentiments may influence from the extensively inclusive to the profoundly exclusive.
Legal immigration includes immigrants, i.e., persons seeking a stable residence card or "green card," and non-immigrants, persons seeking provisional entry to the U.S. under a non-immigrant visa category. Immigration also includes asylum, naturalization, and denaturalization, deportation, as well as immigration crimes, including illegal immigration, or strangers who enter the U.S. and obtain work illegally.
Immigration law concerning the citizens of a country is regulated by international law. The United Nations International Covenant on Civil and Political Rights authorizations that all countries permit admission to its own citizens.
Some countries may preserve rather rigorous laws which control the right of admission and internal rights once established: such as the period of stay, the right to participate in government. Many countries have laws which indicate a process for naturalization, by which immigrants may become citizens.