Why can't they call it a prototype or experiment? Sheesh.

Prototype is usually something working, while spike can be pure research (e.g. validate that APIs are feasible and enough, or that something satisfies requirements). Prototype is usually more polished/usable. Experiment in my mind is something which user-facing but not in stable yet.

So for me they are differentiated enough, but could be that I am just used to it.