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. Wikipedia

A stage before the emergence of language proper Wikipedia Disambiguation

Hypothetical or reconstructed language from which a number of known languages are believed to have descended in historical linguistics. Wikipedia Disambiguation

Common ancestor of a language family Wikidata

A language that in the tree model of historical linguistics is a hypothetical, or reconstructed, typically extinct language from which a number of attested, or documented, known languages are believed to have descended by evolution, by slow and mostly gradual modification, into languages that form a known and documented language family. OmegaWiki

A language which is reconstructed by examining similarities in existing languages to try to deduce what a common ancestor language, no longer known, would have been like. Wiktionary

Common ancestor language. Wiktionary (translation)