If I recall correctly from my high school latin studies words ending with -ior meant to express a property is more of something compared to others, like superior. So junior meant to be younger.
That's why I'm baffled with medior.
Since English is English and not Latin by tradition it might be a "legit" word. Since I'm not a native English speaker I don't know. So...
Is using medior to describe mid-level expertise is right or wrong?