A visa, is a conditional authorization given by a competent authority of a country for a person who is not a citizen of that country to enter its territory and to remain there for a limited duration. Each country typically attaches various conditions to their visas, such as duration of stay, the territory covered by the visa, dates of validity, whether the visa is valid for more than one visit, etc. Visas are associated with the request for permission to enter a country, and are thus, for some countries, distinct from actual formal permission for an alien to enter and remain in the country. In any event, a visa is subject to permission of an immigration official at the time of actual entry, and can be revoked at any time. The visa is commonly a stamp endorsed in the applicant's passport or other travel document. The visa, when required, was historically granted by an immigration official on a visitor's arrival at the frontiers of a country, but increasingly today a traveller wishing to enter another country must apply in advance for a visa, sometimes in person at a consular office, by mail or over the internet.