The term is imprecise as it is generally a marketing term as opposed to a standard.
Gartner defines them thus:
Next-generation firewalls (NGFWs) are deep-packet inspection firewalls that move beyond port/protocol inspection and blocking to add application-level inspection, intrusion prevention, and bringing intelligence from outside the firewall. An NGFW should not be confused with a stand-alone network intrusion prevention system (IPS), which includes a commodity or nonenterprise firewall, or a firewall and IPS in the same appliance that are not closely integrated.
Source: https://www.gartner.com/it-glossary/next-generation-firewalls-ngfws
For what it's worth, Cisco defines five criteria unique to NGFWs. They are listed and described here:
https://www.cisco.com/c/dam/en/us/products/collateral/security/next-gen-firewall.pdf
