confidential 
英 [ˌkɒnfɪˈdɛnʃəl]  美 [ˌkɑnfəˈdɛnʃəl] 
adj. 秘密的；表示信任的；获信任的

例句：
She accused them of leaking confidential information about her private life. 

她指责他们泄露其私生活的秘密。

We'll take good care and keep what you've told us strictly confidential, Mr Lane. 

莱恩先生，我们会非常小心，对你告诉我们的事严格保密。

The envelope was marked `Confidential'. 

信封上标有“机密”字样。

His face suddenly turned solemn, his voice confidential. 

他的脸色突然变得严肃，说话声音也轻悄悄的。

`Look,' he said in a confidential tone, `I want you to know that me and Joey are cops.' 

“听着，”他用隐秘的口吻说，“我想让你知道我和乔伊是警察。”