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.