source: MAL's Secret Society - Forums - MyAnimeList.net

not known or seen or not meant to be known or seen by others:

秘密の, 内密の; 【人に】知られていない, 内緒で «from» ; 〖名詞の前で〗人目につかない〈場所〉

e.g. how did you guess I had a secret plan?

e.g. the resupply effort was probably kept secret from Congress.

attributive not meant to be known as such by others:

比較なし 〖名詞の前で〗人目を避ける, 隠れた〈感情など〉; 内奥の

e.g. a secret drinker.

fond of or good at keeping things about oneself unknown:

〖通例be ～〗 〈人が〉 «…について» 口がかたい «about» ; こそこそやる

e.g. he can be the most secret man.

US (of information or documents) given the security classification above confidential and below top secret.

something that is kept or meant to be kept unknown or unseen by others:

秘密, 機密, 内緒事

e.g. a state secret

e.g. at first I tried to keep it a secret from my wife.

something that is not properly understood; a mystery:

〖通例～s〗 不思議, 神秘, 謎(mystery)

e.g. I'm not trying to explain the secrets of the universe in this book.

a valid but not commonly known or recognized method of achieving or maintaining something:

〖通例the ～〗 «…の» 秘訣; 秘伝, こつ «of, to»

e.g. the secret of a happy marriage is compromise.

formerly, the name of a prayer said by the priest in a low voice after the offertory in a Roman Catholic Mass.

be among the few people who know something.

without others knowing.

make something perfectly clear.

late Middle English: from Old French, from Latin secretus (adjective) ‘separate, set apart’, from the verb secernere, from se- ‘apart’ + cernere ‘sift’.