‘密友’ literally combines 密 (mì), meaning 'close', 'intimate', or 'confidential', and 友 (yǒu), meaning 'friend'. Together, they form a noun that refers to someone with whom you share deep trust, emotional closeness, and mutual understanding—often confiding personal thoughts or secrets. Unlike general terms like 朋友 (péngyou), 密友 implies a higher degree of intimacy and long-standing familiarity.

This word is commonly used in both spoken and written Chinese, especially when emphasizing emotional depth rather than mere frequency of interaction. It’s appropriate in formal writing (e.g., essays about relationships) and everyday conversation among adults. While not inherently formal or literary, it carries slightly warmer, more heartfelt connotations than synonyms like 好朋友, and is rarely used for casual or newly formed friendships.