What is an airport transit visa?

This is the visa you need to transfer through a country. Even if you’re not planning to leave the airport, you may still need a visa. Some reasons include:

If you need a visa to enter a country, then you should assume (until proven otherwise) that you will need a visa to transit through that country.

As the passenger, it is your responsibility to ensure you have the correct visas and travel documents for your trip. Contact the embassy or your foreign ministry for detailed visa information.

Wikipedia is a good source of quick information for visa requirements. For example, here you can see the visa requirements for Indian citizens.