(Gartner is an American company engaged in information technology research and consulting. Its headquarters are located in Stamford, Connecticut. Gartner introduced that in order to deal with the new generation of network security threats at that time and in the future, the firewall must be upgraded to "next-generation firewall" again. This article is Gartner's definition of the next-generation firewall, which is worth a look.
How Gartner Defines Next-Generation Firewalls - SinoGrid.pdf)