A visa is conditional authoritiezation granted by a country to a noncitizen to enter and temporarily remain within that country.
Visas typically include limits on the duration of the duration of the noncitizen's stay, Territory within the country they may enter, the dates they may enter, or the number of permitted visit.
Visas are associated with the request for permission to enter a country and thus are, in some countries district from actual formal permission for an alien to enter and remain in the country.
In each instance, a visa is subject to permission by an immigration official at the time of actual entry and can be revoked at any time.