A visa (short for the Latin carta visa, lit. "the document having been seen") is a document issued by a country giving a certain individual permission to formally request entrance to the country during a given period of time and for certain purposes (see below for caveats and exceptions). Most countries require possession of a valid visa as a condition of entry for foreigners, though there exist exemption schemes (see passport for examples of such schemes). Visas are typically stamped or attached into the recipient's passport, or are sometimes issued as separate pieces of paper.
Visas are associated with the request for permission to enter (or exit) a country, and are thus, for some countries, distinct from actual formal permission for an alien to enter and remain in the country. While a visa for the European Schengen area constitutes the formal permission to enter, according to the conditions, in other cases a visa does not guarantee admission into the country for which the alien has the visa. This formal permission is typically granted by stamping the visa and, in some cases, by providing an additional document as proof of status, such as the United States' I-94. The common phrase "he has to leave because his visa has expired" is thus, strictly speaking, incorrect: the visitor's status has expired, the visa may or may not have.
Some countries, such as some states of the former Soviet Union, require that their citizens, and sometimes foreign travelers, obtain an exit visa in order to be allowed to leave the country. Until 2004, foreign students in Russia were issued only an entry visa on being accepted to University there, and had to obtain an exit visa to return home. This policy has since been changed, and foreign students are now issued multiple entry (and exit) visas. Citizens of the People's Republic of China who are residents of the mainland are required to apply for special permits in order to leave the mainland, including to enter the Special Administrative Regions of Hong Kong and Macao (and SAR residents require a Home Return Permit to visit the mainland). Saudi Arabia requires all resident foreigners, but not citizens, to obtain an exit visa before leaving the kingdom.
Visas are associated with the request for permission to enter (or exit) a country, and are thus, for some countries, distinct from actual formal permission for an alien to enter and remain in the country. While a visa for the European Schengen area constitutes the formal permission to enter, according to the conditions, in other cases a visa does not guarantee admission into the country for which the alien has the visa. This formal permission is typically granted by stamping the visa and, in some cases, by providing an additional document as proof of status, such as the United States' I-94. The common phrase "he has to leave because his visa has expired" is thus, strictly speaking, incorrect: the visitor's status has expired, the visa may or may not have.
Some countries, such as some states of the former Soviet Union, require that their citizens, and sometimes foreign travelers, obtain an exit visa in order to be allowed to leave the country. Until 2004, foreign students in Russia were issued only an entry visa on being accepted to University there, and had to obtain an exit visa to return home. This policy has since been changed, and foreign students are now issued multiple entry (and exit) visas. Citizens of the People's Republic of China who are residents of the mainland are required to apply for special permits in order to leave the mainland, including to enter the Special Administrative Regions of Hong Kong and Macao (and SAR residents require a Home Return Permit to visit the mainland). Saudi Arabia requires all resident foreigners, but not citizens, to obtain an exit visa before leaving the kingdom.

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