秘密 (mìmì) means 'secret' — something deliberately kept hidden from others. The character 秘 (mì) conveys concealment, mystery, or privacy, while 密 (mì) means 'close', 'tight', or 'dense', suggesting something tightly guarded or not easily penetrated. Together, they emphasize both the intentional hiding and the impenetrable nature of the information.

This noun is commonly used in personal, social, and professional contexts — for example, a child’s hidden wish, a friend’s confidential confession, or classified company data. It can refer to facts, plans, feelings, or objects intentionally withheld. Unlike the English word 'secret', 秘密 is almost always neutral or slightly positive; it rarely carries sinister connotations unless context specifies otherwise (e.g., 国家秘密 'state secret'). It functions as a countable noun: you can say 一个秘密 (yī gè mìmì, 'a secret') or 很多秘密 (hěn duō mìmì, 'many secrets').