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In the tree model of historical linguistics, a proto-language is a postulated ancestral language from which a number of attested languages are believed to have descended by evolution, forming a language family. Proto-languages are usually unattested, or in some cases only partially attested. They are reconstructed by way of the comparative method.
In the family tree metaphor, a proto-language can be called a mother language. Occasionally, the German term Ursprache ] is used instead. It is also sometimes called the common or primitive form of a language.
In the strict sense, a proto-language is the most recent common ancestor of a language family, immediately before the family started to diverge into the attested daughter languages. It is therefore equivalent with the ancestral language or parental language of a language family.
Moreover, a group of languages which are not considered separate languages may also be described as descending from a unitary proto-language.