Rare standalone noun; mostly encountered in fixed expressions like 秘中の秘 (secret of secrets).
See also: 秘中の秘
It is sometimes used in the expression 秘中の秘 (secret of secrets).
secretary; private secretary · treasured book; secret book
constipation
mystery; mysteriousness; hidden wonder
hide; keep inside; conceal
secret; key; trick; recipe
top secret; absolute secrecy
secret story; untold episode
unexplored region; secluded place; off the beaten track
hidden treasure; secret treasure
hiding; concealment
secret; mystery; esoteric knowledge
treasure; cherish; prize; hold dear
keeping a secret; secrecy; confidentiality
remaining silent; keeping secret
secret art; secret technique; mysteries of an art
secret plan; secret strategy
secret medicine; secret remedy; nostrum · wonder drug; excellent remedy
private records; secret notes; confidential papers
secret method; secret formula · esoteric rituals (Shingon Buddhism)
secret technique; secret art
hidden Buddhist image; secret Buddha statue
sacrament
favorite child; favorite student; apple of one's eye
company secret; confidential (outside the company)
keeping secret; confidentiality
strict secret; top-secret
secrecy; confidentiality
in secret; secretly; confidentially
secret; secret matter
The kanji 秘 means 'secret' or 'hidden'. The reading ひ is the on'yomi. The standalone noun use is rare and mostly found in classical or fixed expressions.