In historical linguistics, a daughter language, also known as descendant language, is a language descended from another language, its mother language, through a process of genetic descent. If more than one language has developed from the same proto-language, or 'mother language', those languages are said to be sister languages, members of the same language family. These concepts are linked to the tree model of language evolution, in which the relationships between languages are compared with those between members of a family tree. This model captures the diversification of languages from a common source.

Strictly speaking, the metaphor of the mother-daughter relationship can lead to a misconceptualization of language history, as daughter languages are direct continuations of the mother language, which have become distinct, principally by a process of gradual change; the languages are not separate entities "born" to a parent who eventually dies.

Mother languages do not "die", they become their daughter languages.