VISA is a permission or an authorization given on passport specifying that the holder is permitted to enter, leave or stay in a country for a specified period of time.
Two Important Points:
Here, two points are important. First, a visa is permission to enter or stay in a country for a specified period of time, and second, authorization is given on your passport.
And for getting a visa you are expected to go through an entire process and sometimes you are also expected to pay some fee for that.
What Is Transit Visa?