visa is the usually a stamp in a passport allowing entry into a country for a specific purpose and a finite amount of time
Other definition:
Visa is the formal approval and permission for entry into a given country. Usually issued by the destination country's consular offices or embassy. Visa formats vary, from simple ink stamps in the passport to holographic images with photos stapled to the passport.
Other definition:
Visa is the stamp of approval recorded in a traveler"s passport to enter a country for a specific purpose.
The Travel-Glossary: Everything about Travelling, Sightseeing and the World.