Simply "visa." It is not a proper noun, so it is not capitalized when not at the beginning of a sentence. However, you may see the term "Visitor Visa" capitalized on a form or a website. This is not correct, but sometimes people like to capitalize the names of official documents and things like that (Driver's License comes to mind as something that one sometimes sees capitalized even though it shouldn't be). You may also see the "Visa" capitalized when it refers to a particular brand of credit card, which is used in the US and elsewhere. In this case, Visa is a proper noun because it is the name of a company. The logo of the company includes the word VISA in all capital letters, but you would write "Visa" if you were writing about it in a sentence. Again, the travel document is simply "visa."