Visa is the annotation and seal of the sovereign organ of a country on the passport or other travel documents held by its own or foreign citizens to show that they are allowed to enter or leave its own territory or pass through it. It can also be said that it is an endorsement certificate issued to them. Generally speaking, a visa is a document issued by a country's entry-exit administration department (such as immigration bureau or its embassy or consulate abroad) to express approval to foreign citizens.
The visa is usually attached to the passport or other international travel documents held by the applicant. Under special circumstances, valid passports or other international travel documents can be written on a separate sheet. With the progress of science and technology, some countries began to issue electronic visas and biological visas, which greatly enhanced the anti-counterfeiting function of visas.
Visa plays an important role in checking and controlling entry-exit personnel, protecting national security and preventing illegal immigrants and criminals.
Passport is the holder's nationality and identity certificate, and visa is the permission certificate of a sovereign country to allow foreign citizens or their own citizens to enter, exit or transit.
Visas are generally endorsed on passports, some are endorsed on other travel documents instead of passports, and some are issued separately. For example, immigration visas in the United States and Canada are A4 paper, and Singapore also issues separate paper visas to foreigners. Generally speaking, a visa must be used at the same time as a passport to be valid.