A service that helps merchants to initiate ecommerce, in-app and point-of-sale payments. It is not directly involved in the money flow; typically, it is a web server to which a merchant's website or a POS system is connected.
A payment gateway can be provided by a bank, or can exist as a separate service that connects to one or more payment processors. Adyen combines the functions both of a payment gateway and a payment processor, as well as a risk management system and an acquirer. This makes it a full-stack payment service provider.