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 idioms (such as a dialect cluster) which are not considered separate languages (for whichever reasons) can also be described as descending from a unitary proto-language.
Occasionally, the German term Ursprache (from Ur- "primordial" and Sprache "language", pronounced ) is used instead.
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".