secret art; secret technique; mysteries of an art
Refers to a closely guarded skill, method, or esoteric knowledge, often in traditional arts, crafts, or martial disciplines.
その流派の秘術は門外不出とされている。
The secret techniques of that school are said to never be revealed to outsiders.
He was initiated into the secret art by his master.
numerology
Compound of 秘 (secret) and 術 (technique, art).