An advanced person; someone with a high level of skill or knowledge, beyond beginner or intermediate level.
このクラスは上級者向けなので、経験者のみ参加してください。
This class is for advanced students, so only experienced people should join.
彼はプログラミングの上級者で、複雑なアルゴリズムも実装できる。
He is an advanced programmer and can implement complex algorithms.
上級者と初心者では求められる練習量が違う。
The amount of practice required differs between advanced learners and beginners.