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。

He could be a confidential agent, she speculated. Or a private investigator. 

他可能是一个秘密特工，她推测道。或者是一名私家侦探。