Esoteric

Designed for or understood by the specially initiated alone.

Learned borrowing from Ancient Greek ἐσωτερικός (esōterikós, “belonging to an inner circle”), from ἐσωτέρω (esōtérō, “further inside”), comparative of ἔσω (ésō, “within”), from ἐς (es), εἰς (eis, “into”) (the term esoteric referred originally to the secret teachings of Greek philosophers, versus public or exoteric ones).

arcane, recondite, acroamatic