Linguistic reconstruction is the practice of establishing the features of an unattested ancestor language of one or more given languages. Wikipedia

processes of understanding how earlier languages were spoken Wikidata

An unattested language or linguisitc entity that is described as the result of research on the features of one, or several related, attested languages by which scientist with some precision find out what the common language looked like that preceeded them in history. OmegaWiki