What is a transit visa, and when do you need one?
Permission to pass through a country on the way to somewhere else, which is required in more situations than travellers expect — and discovering you need one at the departure gate is a common and expensive way to miss a trip.
When you may need one:
Leaving the airport during a layover, which almost always requires entry permission of some kind.
Changing airports in the same city, which counts as entering the country.
Collecting and re-checking baggage, which frequently requires passing through immigration even if you never leave the terminal.
Changing from an international to a domestic flight, which necessarily involves entering.
Certain nationalities, who require a transit visa even for an airside connection without leaving the sterile area — this is the case that surprises people most, and it is nationality-specific rather than route-specific.
Overnight connections, where there is no airside accommodation.
When you generally do not need one: a short airside connection in the same terminal, with baggage checked through, on a single ticket, holding a nationality not subject to airside transit requirements.
The specific traps:
Separate tickets. If your journey is on two unconnected bookings, baggage will almost certainly not transfer, so you must collect it and check in again — which means entering the country, requiring permission and enough time.
Electronic authorisations. Several countries require a pre-travel authorisation rather than a visa, obtained online, and it applies to transit as well as to entry.
Visa requirements for your destination being checked by the airline at departure — carriers are fined for bringing inadmissible passengers, so they will refuse boarding rather than risk it.
Connecting through your own country of citizenship, which occasionally creates its own requirements.
What to do: check the transit rules for your nationality, for the specific airport, and for your exact routing; check whether your tickets are on one booking; allow substantially more time if you must clear immigration; and verify directly with the authority of the transit country rather than relying on a booking site.
General information; rules change and vary by nationality.