hidden mysteries; secret principle
Refers to esoteric or profound truths that are not openly revealed, often in religious, philosophical, or mystical contexts. The term is somewhat literary and not an everyday word.
That scripture expounds the hidden mysteries of the universe.
After years of training, he finally realized that secret principle.
Compound of 秘 (secret, hidden) and 鍵 (key). The exact historical derivation is uncertain, but the term likely emerged in Buddhist or esoteric contexts to denote a key to hidden truths.