The stuff that gets your desktop started up into a usable state. Same as on any OS, macOS or Windows. The software that is expected to be there on a given version of the OS, that other software may depend on being at some particular (major, at least) version until there’s a new version of the OS.
The stuff that updates when you update the OS.
On macOS, the software that’s not an optional package manageable through the App Store (even if installed by default) or managed by Homebrew or what have you.
This is a distinction that should NOT exist. Like on phones, you end up with "software" that is just a glorified web browser and doesn't integrate with the rest of your system nor cannot access your hardware to its full extent.
I.e. If I want to set up a script that makes Libreoffice trigger phone calls through a bluetooth modem, I should be able to. Otherwise it isn't really a computer. These "system" vs "non-system" almost always end up down this slippery slipe, and avoiding it is one of the reasons I enjoy desktop Linux for all its brokenness.
It’s very nice when it exists because you can do whatever you need with user-facing software without risking system stability. Long-term stable versions of basic software, including gui libs, also provides a reliable target for software deployment.
I've made up half a dozen definitions for it before asking, and none of them were a good guide to decide about the software on my machine. Yours seems to focus on the DE components, what is both way too restrictive (why are `ls` or `test` out?) and way too inclusive (my DE installs with an Earth rendering and graphic calculator, my workplace's DE installs with CandyCrush).
A universal definition isn’t needed to apply the concept, in the same way the Internet can’t agree on some fixed universal definition for what a sandwich is, yet this doesn’t impair assembling a BLT.
It is in fact applied by organizations, and they manage fine, so lack of a universal definition isn’t a hindrance.
If you want to guarantee certain versions of ls or test are available for the duration of the supported life of an OS, yeah, they’re part of the base system. This kind of arrangement is very nice for both users and software vendors. The base-system instability of an Arch or a Gentoo (rolling release), or the ancient productivity-software packages of a Debian, aren’t the only options—lockstep-release stable base system and rolling release user packages are an option.
