The noun 秘密 (mìmì) means 'secret.' It can also function as an adjective meaning 'secret' or 'confidential.' For example, 这是一个秘密 means 'This is a secret,' and 秘密项目 means 'a secret project.'
我告诉你一个秘密,你别跟别人说。
Wǒ gàosù nǐ yī gè mìmì, nǐ bié gēn biérén shuō.
I'll tell you a secret — don't tell anyone else.
她心里藏着一个不能说的秘密。
Tā xīnlǐ cáng zhe yī gè bù néng shuō de mìmì.
She keeps a secret in her heart that cannot be told.
这件事是我们之间的秘密。
Zhè jiàn shì shì wǒmen zhījiān de mìmì.
This matter is a secret between us.