Technical term in historical linguistics for a reconstructed ancestral language from which a family of languages is derived.
Proto-Indo-European is considered the protolanguage of many modern languages.
祖語 is a more common term for 'proto-language' or 'ancestral language'. 共通基語 is a less frequent, more technical synonym.
Compound of 共通 (common) and 基語 (base language), likely a calque from English 'protolanguage' or German 'Ursprache'.