A visa (from the Latin charta visa, lit. "paper that has been seen") is an indication that a person is authorized to enter the country which "issued" the visa, subject to permission of an immigration official at the time of actual entry. The authorization may be a document, but more commonly it is a stamp endorsed in the applicant's passport. Some countries do not require a visa in some situations, such as a result of reciprocal treaty arrangements. The country issuing the visa typically attaches various conditions to the visa, such as the time that the visa is valid, the period that the person may stay in the country, whether the visa is valid for more than one visit, etc. The possession of a visa is not in itself a guarantee of entry into the country that issued it, and a visa can be revoked at any time.

A visa does not generally give a person any rights beyond the right to enter a country and remain there. Anything beyond those basic entitlements requires special permits, such as a residency permit or work permit.

Many countries require possession of a valid passport and visa as a condition of entry for foreigners, though there exist exemptions (see below for examples of such schemes).

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.

Some countries require that their citizens, and sometimes foreign travelers, obtain an exit visa in order to be allowed to leave the country.