[moe]

No.

I mean an isolated unit that could be ripped out at any time and would be immediately useful on its own.

Anyway, abecedarius (below) has phrased it better than I could.

[Shorel]

Those 'isolated units' are commonly called libraries then.

[moe]

No. They are called "module".

[Shorel]

Yes, I agree with that name, libraries usually start as modules and some extra work is required to convert a module to a library.

Also, a library can be thought of as a collection of modules.