保密(bǎo mì) - to keep secret, to maintain confidentiality in Mandarin

保密 (bǎo mì) is the verb for keeping something confidential. It covers everything from a friend's private news to a state secret, and it is the standard word on official documents and workplace contracts.

In use it usually takes 对 for the thing being protected: 对这件事保密 means to keep quiet about this matter. It can also stand alone as an instruction, and 请保密 on the end of a message is the Mandarin equivalent of "this is between us".

The trap for English speakers is treating 保密 as a noun. It is a verb, so "keep the secret" is 保密, not 保守一个保密. If you want the noun, the word you want is 秘密 (mì mì), the secret itself.