Payment gateway

A payment gateway is the service that sits between a merchant's checkout and the card networks, doing the unglamorous work of tokenising the card, routing the transaction through the acquirer, handling 3DS challenges, and returning an approve/decline. Stripe, Adyen, Braintree, and PayPal Braintree are the canonical examples; Stripe's API is the reason most developers know the category at all. Two things a gateway has to solve that aren't obvious: PCI DSS compliance so the merchant never touches raw card data, and fraud detection so false declines don't kill conversion. Mental model: the gateway turns "accept payments" from a 9-month bank integration into a 10-line SDK call.