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 (pronounced [ˈuːɐ̯ʃpʁaːxə] ^ⓘ; from ur- 'primordial, original' + Sprache 'language') 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 lects that are not considered separate languages, such as the members of a dialect cluster, 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. Typically, credibility is given to the hypotheses of highest compatibility. The differences in compatibility must be explained by various applications of the wave model. The level of completeness of the reconstruction achieved varies, depending on how complete the evidence is from the descendant languages and on the formulation of the characters by the linguists working on it. Not all characters are suitable for the comparative method. For example, lexical items that are loans from a different language do not reflect the phylogeny to be tested, and, if used, will detract from the compatibility. Getting the right dataset for the comparative method is a major task in historical linguistics.

Some universally accepted proto-languages are Proto-Afroasiatic, Proto-Indo-European, Proto-Uralic, and Proto-Dravidian.