noun
advanced learner; experienced person
Refers to someone with a high level of skill or knowledge in a particular field, often contrasted with beginners (初心者) or intermediate learners (中級者).
このコースは上級者向けです。
This course is for advanced learners.
彼はプログラミングの上級者だ。
He is an advanced practitioner of programming.
中級者 means 'intermediate learner', sitting between 初心者 and 上級者.
Compound of 上級 (じょうきゅう, 'advanced level') and 者 (しゃ, 'person').