1. secret: if something is secret , not many people know about it, and they agree not to tell anyone else about it

例：

The president's schedule is secret, but there is speculation that he will visit UN troops in the area.（总统行程是保密的，但有人猜测他会去慰问驻扎在当地的联合国部队。）

remain/stay secret

I hope you see how vital it is that our conversation remains secret.（我们俩所说的要保密，我希望你明白这点有多重要。）

top secret (=when an official document, piece of information etc. is very secret)

The experiments were top secret.（这些实验是高度机密。）

2. confidential: confidential information is known only by a few official people, and must not be told to anyone else, for example because it contains military secrets or private details about people例：

a confidential government report（政府机密报告）

Always protect confidential files by locking them with a password.（机密文件一定要加密保管。）

highly/strictly confidential (=very confidential)

The information we received is of a highly confidential nature and relates to national security.（我们收到的这个信息关系国家安全，要高度保密。）

The information will be regarded as strictly confidential (= completely confidential ) .（这个信息一定要高度保密。）

3. classified: classified information or documents are ones that the government has ordered to be kept secret

例：

There is evidence that Huang had access to classified information.（有证据表明，黄某已经得到了机密信息。）

4. sensitive: information or documents that are sensitive are likely to cause problems or embarrassment if they are made public and therefore they are kept secret

例：

A teenager used his personal computer to break into sensitive US Air Force files.（一个青少年用私人电脑破解了美国空军的敏感文件。）

highly sensitive

The minister admitted that highly sensitive documents had been leaked to the press.（这位部长承认说有人已将政府敏感文件透露给媒体。）

5. covert: done secretly, especially by a government or official organization, often involving breaking the law

例：

convert operations（秘密行动）

6. underground: an underground organization or newspaper is one that operates or is produced secretly and opposes the government

例：

Her father was a member of the underground resistance movement in France during World War II.（二战期间，她爸爸是法国地下抵抗运动的成员。）

7. undercover: working secretly - use this about the activities of the police, the army etc.

例：

Six members of a drug-smuggling gang were arrested after an 18-month undercover police operation.（警方经过18个月秘密侦查，抓获了6名毒贩。）

The unit is equipped to deal with a variety of situations, including undercover surveillance.（这个公寓设施齐全，可以在很多情况下使用，包括进行秘密监视。）

8. hush-hush: used about information or activities that are kept officially secret

备注：''hush-hush''常用于非正式的场合。

例：

His death was really hush-hush. It was so covered up that you wondered if it wasn't a suicide.（他的死讯高度保密，掩藏得这么好，倒让人觉得这可能不是自杀。）

'The Manhattan Project' was the insiders' name for the hush-hush project.（“曼哈顿计划”是这个秘密计划的内部代号。）

He was put in charge of some hush-hush military project.（他被任命负责一个秘密军事项目。）

I've no idea what he does - it's all very hush-hush.（我不了解他是做什么的，总搞得很神秘。）

9. cloak-and-dagger: adj. use this about methods and activities that involve a lot of secrecy, especially when this seems unnecessary

备注：''cloak''原意是“斗篷”，''dagger''原意是“匕首”，''cloak-and-dagger''用于形容“跟间谍有关的”。

例：

He was arrested after a cloak-and-dagger operation involving the CIA and MI6.（他因为涉及间谍活动窃取中央情报局和军情六处的资料而被捕。）