DMZ
Also known as Demilitarized zone
A separate network zone for systems that must be reachable from the internet, isolated from the internal network.
Anything internet-facing lives here so that compromising it does not put the attacker on the internal network. Traffic from the DMZ inward is tightly filtered.
It matters less than it did now that most public services are hosted rather than self-run, but it remains relevant for anyone still publishing an application from their own infrastructure.
Where this comes up
DMZ sits inside our networks and infrastructure work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.