Never thought of "external" (singular) in terms of a "set". Don't you mean "an object" that is not part of Pd Core. By the way, by "Pd Core", do you mean the Pd binary (not including the "extra" files?) or all objects in the Pd Vanilla distribution?

Well, I always thought that a library could have compiled objects, abstractions, or both. I have seen many libraries in Extended that had both compiled and abstractions objects.

And, again, I'm confused with the singular/plural idea. Seems you're thinking that only one abstraction can be a "library", or just a single compiled object. But I've never seen people referring to, for example, the freeverb~ external as the "freeverb~ library". So my idea of "library" is really a collection of objects.

Surely I might have gotten things just wrong and that's why I'm asking if there is a consensus in the terminology

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.

cheers