妹妹 (mèi mei) is a noun meaning 'younger sister' and carries an affectionate, familiar tone. The character 妹 (mèi) on its own means 'younger sister', and reduplication — repeating the same character — softens the term, making it warmer and more intimate, often used in family conversations or when speaking fondly of a younger female sibling. It’s commonly used by older siblings, parents, or even close friends referring to someone’s younger sister.

This term emphasizes age difference and familial closeness rather than formal kinship structure. Unlike the more neutral or formal 姐妹 (jiě mèi, 'sisters') or the strictly biological term 妹 (mèi) alone, 妹妹 implies emotional warmth and everyday familiarity. It’s rarely used in official documents or formal speeches, but appears frequently in casual speech, family chats, and personal storytelling.