Payment Gateway
The service that processes online card payments.
A payment gateway is the service that securely authorises and processes a customer's payment at checkout, moving money from their card to your account. Shopify Payments is the native option; third-party gateways (and regional providers like Paymob or valU) can be added, though using one outside Shopify Payments incurs an extra transaction fee. Processing fees directly affect your per-order profit, so the gateway choice is a margin decision as much as a technical one.