Token showing authorization to apply to enter the territory for which it was issued. Don't forget to include your citizenship when asking!
A visa is a token giving rights to apply to enter or leave a country. Visa are usually attached by pasting a sticker or stamping it in a holders passport.
Visas indicate that the holders thereof may present themselves at a border and request entry into the issuing country. Visas may have both a duration of validity and a visit time limitation.
Duration of validity here means the period over which the visa itself is valid. Typically three months for all types of visa, it is the interval over which the invitation to present oneself at a border is valid. Thereafter the visa, whether or not by then presented, expires.
Visit time limitation, often 15, 30 or 90 days, here means the maximum period of time allowed in country if entry is granted. The person responsible (border control) for allowing or declining entry may decide upon a visit time duration that is less than the maximum available for the relevant visa, or decline entry altogether.
The latter (visit time limitation) is a matter of “What is the longest I may stay if I am permitted entry?”. The former (duration of validity) is a matter of “When is the cut-off for use it or lose it?” For the latter it may be worth bearing in mind that, at least in theory, the visa application process is to permit countries to ban entry to people they consider undesirable. Therefore a time limit is necessary to allow for a change in circumstances, regarding the deemed desirability of a visit by a specific individual, that my take place long after a visa was issued.
“In theory” because in practice visas are essentially just a means to tax prospective visitors, often tit-for-tat in response to the charges imposed by the home country of the would-be visitor on visitors from the country issuing the visa.
The word visa should not be capitalised.
“Days” for visas are calendar basis and not hours basis. For example, noon (local time) to noon is at least two “days”.
A visa that expires “1.1.2020” is still valid until the midnight between January 1 and January 2, 2020.