Well, not “medior” at any rate. That’s a fabrication, an anglicization.
It’s a term used in particular in job descriptions and on recruitment sites for someone with a bit of experience who can’t be called a junior but who they can’t yet justify paying the salary of a senior…
- alternative: mid-level
- alternative: just describe it – “with three to five years’ experience” or something like that
The same with family names (seen more frequently in US English): the dad is Senior, the son is Junior and there ain’t no Medior.
Prevalence: high. CVs, recruitment, job functions and so forth. High enough that there are thousands of hits now on English-language websites. Largely non-native, of course.
Frequency: high. If the author thinks it’s English, the website is going to be liberally peppered with the term.
Native: no. First time I came across it, I genuinely didn’t know what it meant.