A daughter is a female child in relation to her parents. If you have a mother and father, you are their daughter or son. Your sister is your parents' daughter, and if your aunt has a girl, that girl is your aunt's daughter (and your cousin).

The word describes a relationship: the same girl who is a daughter to her parents might also be a sister to her brother, a granddaughter to her grandparents, and a niece to her uncle. She's all of these things at once, depending on who's describing the relationship.

The word isn't just used for kids. Even when daughters grow up and become adults themselves, they remain their parents' daughters.