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 partially attested at best. 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 (from Ur- "primordial, original", and Sprache "language", ˈuːɐ̯ʃpʁaːxə) is used instead. It is also sometimes called the common or primitive form of a language (e.g. Common Germanic, Primitive Norse).
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 (such as a dialect cluster) which are not considered separate languages (for whichever reasons) may also be described as descending from a unitary proto-language.
Typically, the proto-language is not known directly. It is by definition a linguistic reconstruction formulated by applying the comparative method to a group of languages featuring similar characteristics. The tree is a statement of similarity and a hypothesis that the similarity results from descent from a common language.
The comparative method, a process of deduction, begins from a set of characteristics, or characters, found in the attested languages. If the entire set can be accounted for by descent from the proto-language, which must contain the proto-forms of them all, the tree, or phylogeny, is regarded as a complete explanation and by Occam's razor, is given credibility. More recently, such a tree has been termed "perfect" and the characters labelled "compatible".
No trees but the smallest branches are ever found to be perfect, in part because languages also evolve through horizontal transfer with their neighbours.