A stub is a stand-in for an unfinished part of a larger structure. It temporarily completes an unfinished piece of theory, or an unknown value in an equation. Instead of there not being anything at all, a stub is created to fill a void.
One could say a stub fulfils the essence of being, as it is fully defined in relation to its neighbouring other (more advanced and demanding) entities, yet has no identity of its own. It is an inversion of the requirements of all that it's not.
It's debatable whether we prefer a stubbed system, which is usually incorrect, over a system that is always broken.