That being said, I think it boils down to, for me, “external(s)” was short for “external library(-ies),” as in software libraries and, even if there is only 1 class or object, it’s still a “library.”

From what I understand, your coming from the Pd object side so “external” means “1 object” not part of Pd vanilla and “externals” means “multiple objects” not part of vanilla. I think that definitely makes sense and perhaps I was coming from the wrong/developer direction.

So, in short, the idea I've had so far is:

external (kinda short for "external object"): a single object not part of the Pd Vanilla distribution, either an abstraction or a compiled object 

library (kinda short for "externals library"): collection of externals.

Yeah, I think that’s probably a good way to look at it (as mentioned above).

I do like to note when a library is composed completely of abstractions, hence “abstraction library”, as it's easy to use and doesn’t require compilation.