The service that automates the payment transaction between the shopper and merchant. It is usually a third-party service that is actually a system of computer processes that process, verify, and accept or decline credit card transactions on behalf of the merchant through secure Internet connections. The payment gateway is the infrastructure that allows a merchant to accept credit card and other forms of electronic payment. When referring to payment gateways used for Internet transactions, it may also be called an IP payment gateway.
See gateway for further understanding of how a gateway works.