What adjective is between "intermediate" and "advanced"?

You could use Roman numerals to distinguish the different levels, e.g., Intermediate I, Intermediate II.

Intermediate is defined as the state of being between two extremes. In the academic sense, these extremes are beginning and advanced.

: being or occurring at the middle place, stage, or degree or between extremes

Having said that, I suggest high intermediate or intermediate plus.

A few words come to mind: experienced, proficient and skilled. It's hard to distinguish what comes in between those words exactly. I usually see the progression being beginner, intermediate and advanced. But I also feel that those three alone don't make up all levels of knowledge or skill.

Between intermediate and advanced there is "high-intermediate".