秘密 is the common word for 'a secret' (noun) or 'secret/confidential' (adjective). It's more general than 机密 (jī mì), which refers to classified or official secrets, and 隐私 (yǐn sī), which is about personal privacy. As an adjective, 秘密 is used more broadly than 密 (mì), which often means 'dense' or 'close' and appears in fixed compounds.