A visa (from the Latin charta visa, meaning "paper that has been seen") is a conditional authorization granted by a polity to a foreigner that allows them to enter, remain within, or leave its territory. Visas typically include limits on the duration of the foreigner's stay, areas within the country they may enter, the dates they may enter, the number of permitted visits, or if the individual has the ability to work in the country in question. Visas are associated with the request for permission to enter a territory and thus are, in most countries, distinct from actual formal permission for an alien to enter and remain in the country. In each instance, a visa is subject to entry permission by an immigration official at the time of actual entry and can be revoked at any time. Visa evidence most commonly takes the form of a sticker endorsed in the applicant's passport or other travel document but may also exist electronically. Some countries no longer issue physical visa evidence, instead recording details only in immigration databases.

Historically, immigration officials were empowered to permit or reject entry of visitors on arrival at the frontiers. If permitted entry, the official would issue a visa, when required, which would be a stamp in a passport. Today, travellers wishing to enter another country must often apply in advance for what is also called a visa, sometimes in person at a consular office, by post, or over the Internet. The modern visa may be a sticker or a stamp in the passport, an electronic record of the authorization, or a separate document which the applicant can print before entering and produce on entry to the visited polity. Some countries do not require visitors to apply for a visa in advance for short visits.

Visa applications in advance of arrival give countries a chance to consider the applicant's circumstances, such as financial security, reason for travel, and details of previous visits to the country. Visitors may also be required to undergo and pass security or health checks upon arrival at the port of entry.

Some polities which restrict emigration require individuals to possess an exit visa to leave the polity. These exit visas may be required for citizens, foreigners, or both, depending on the policies of the polity concerned. Unlike ordinary visas, exit visas are often seen as an illegitimate intrusion on individuals' right to freedom of movement. Imposition of an exit visa requirement may be seen to violate customary international law, as the right to leave any country is provided for in the Universal Declaration of Human Rights.

Uniquely, the Norwegian special territory of Svalbard is an entirely visa-free zone under the terms of the Svalbard Treaty. Some countries—such as those in the Schengen Area—have agreements with other countries allowing each other's citizens to travel between them without visas. The World Tourism Organization announced that the number of tourists requiring a visa before travelling was at its lowest level ever in 2015.