A visa is a conditional authorization granted by a country to a foreigner, allowing them to enter and temporarily 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.(from Wikipedia)