Best answer
In general, 비밀 (비밀 / himitsu ) is the commonly used word.
Example usage would be
비밀의 문 (문) / himitsu no tobira (secret door) 가족 (가족)의 비밀 / kazoku no himitsu (family secret) 성공 (세이코 )의 비밀 / seikou no himitsu (secret (s) to success)
The word 비밀 has a bigger, generalized / longer lasting feel to it.
If you are talking about a personal secret, then 비밀 (실마리 / naisho ), meaning “for your eyes only” or “[in] confidence / secret”is often used.
Eg
이것은 비밀 (실마리) 야. / kore wa naisho dayo (please keep this a secret) 비밀의 이야기 (이야기) / naisho no hanashi (secret conversation) 몰래 담배를 피우는 (산수) / naisho de tabako wo suu (smoke in secret)
The word 비밀 has less formal / a little more personal / temporary feel to it.
If it s an institutional secret, eg a government or military secret, then 기밀 (미쓰 / kimitsu ) is used.
기밀 문서 (미쓰 문서) / kimitsu bunsho (confidential / classified document)
So if I were asking someone to keep something a secret for time being in a friend-to-friend conversation, I d say :
“이것은 비밀 해 놓고. “/ kore wa naisho ni shite oite (please keep this a secret for now.)
vs. If I were in an office, and talking about something I want to keep under wraps :
“이것은 이마까지 비밀이지만, 사토 (사토) 씨는 전근 (天金)한다고합니다 “ Kore wa ima no tokoro himitsu desuga, Sato-san wa tenkin suru soudesu. (This is a secret for now, but Mr./Ms. Sato will be transferred to another office)
I hope this answered your question – if more clarification is needed, please let me know.
Answer
Being the rich language that Japanese is, you get several variations. Here are a few …
괴물 bakemono or 유령 obake They literally mean “something that changes its shape”or a “shapeshifter”The reason for bringing these words up first is that bakemono is used commonly with any kind of beastly creature , and obake is often used for apparitions such as ghosts or specters, but they are really referring to living creatures such as a tanuki (raccoon) or kitsune (fox). These fall under the category of youkai. (See below)
괴물 kaibutsu This also means monster, and includes both Japanese and foreign countries monsters such as the Frankenstein monster or the Creature from Black Lagoon.
괴수 kaijuu This is used mainly for huge monsters like Godzilla or Mothra.
요괴 youkai It is a term that encompasses most monsters of Japanese folklore such as the Tengu or Kappa, which have animal features. Other youkai can have human appearance, such as the rokurokubi or nopperabou (shapeshifters), and some can conceal themselves as regular household objects.
몬스터 Monstaa This is the katakana version of the Japanese pronounciation of the English word “monster”