Payment gateway failover is the most common form of payment failover, which covers the broader idea of keeping payments working when any part of the chain breaks. This page explains the narrower, gateway-level meaning.
A payment gateway passes card and other payment requests from a merchant's checkout to an acquirer or processor. If that gateway suffers an outage, slows down sharply or starts returning errors, every sale can fail at once. Gateway failover means the merchant's systems detect the problem, for example through timeouts or a rise in error codes, and route new transactions to a second gateway or processor that has already been set up and tested. Done well, the customer never notices.
Failover requires preparation. The merchant needs a contract and integration with more than one provider, card details stored in a form both can use, such as network tokens rather than a single gateway's proprietary tokens, and rules that avoid charging a customer twice if the first request actually succeeded. Many merchants use a payment orchestration platform to manage this, often together with multi-acquirer processing. Failover is about availability; choosing the best provider for each transaction when everything is working is the separate job of payment routing.