What is between a rookie and a veteran?

Depends on what you are talking about.

If you are referring to baseball, probably a seasoned player.

In other things, it is referred to as an amateur, which refers to practiced but not professional (that is what they call a civilian ham radio operator).

Occasionally it would be called a professional.

Other options could include words like "established," "seasoned," "practiced," etc.

"Intermediate" comes to mind.

A seasoned player

A regular xD