Is it a zone that is inside the firewall? i.e., the zone that the firewall protects? I have read information from zenarmor, barracuda, and they have left me even more confused.

demilitarized zone. its the network (LAN) behind the firewall.

Personally I dislike this term since ages.

Yes, in the end it just describes a "zone" and back in the old days most if not all small scale deployments just had wan, lan, and a server network.

But what actually matters are rules between the zones. And again back in the days most firewall rules where simply, disallow wan to lan, but trust all lan to "dmz"/server. And between server and lan? Good luck -.-

Tldr old farts rant: please just all it all a "firewall". Be it zone based or otherwise organized....