Visa is a conditional authorization granted by a country to a foreigner, allowing them to enter, remain within, or to leave that country. Visas typically include limits on the duration of the foreigner's stay, territory within the country they may enter, the dates they may enter, the number of permitted visits or an individual's right to work in the country in question. Visas are associated with the request for permission to enter a country and thus are, in some 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. A visa most commonly takes the form of a sticker endorsed in the applicant's passport or other travel document.
The main reasons states impose visa restrictions on foreign nationals are to curb illegal immigration, security concerns, and reciprocity for visa restrictions imposed on their own nationals. Typically, nations impose visa restrictions on citizens of poorer countries, along with politically unstable and undemocratic ones, as it is considered more likely that people from these countries will seek to illegally immigrate. Visa restrictions may also be imposed when nationals of another country are perceived as likelier to be terrorists or criminals, or by autocratic regimes that perceive foreign influence to be a threat to their rule. According to Professor Eric Neumayer of the London School of Economics:
"The poorer, the less democratic and the more exposed to armed political conflict the target country is, the more likely that visa restrictions are in place against its passport holders. The same is true for countries whose nationals have been major perpetrators of terrorist acts in the past".
A fee may be charged for issuing a visa; these are often also reciprocal—hence, if country A charges country B's citizens US$50 for a visa, country B will often also charge the same amount for country A's visitors. The fee charged may also be at the discretion of each embassy. A similar reciprocity often applies to the duration of the visa (the period in which one is permitted to request entry of the country) and the number of entries one can attempt with the visa. Other restrictions, such as requiring fingerprints and photographs, may also be reciprocated. Expedited processing of the visa application for some countries will generally incur additional charges.
Government authorities usually impose administrative entry restrictions on foreign citizens in three ways - countries whose nationals may enter without a visa, countries whose nationals may obtain a visa on arrival and countries whose nationals require a visa in advance. Nationals who require a visa in advance are usually advised to obtain them at a diplomatic mission of their destination country. Several countries allow nationals of countries that require a visa to obtain them online.