What is the hypothetical proto-language from which all Indo-European languages descended called?

The hypothetical proto-language from which all Indo-European languages descended is called Proto-Indo-European.

Proto-Indo-European, commonly abbreviated PIE, is not preserved in a surviving written document. Linguists reconstruct it by comparing patterns in historically documented languages, including Sanskrit, Ancient Greek, Latin, Gothic, and Old Church Slavonic. Shared vocabulary, sound correspondences, and grammatical features reveal relationships that cannot plausibly be explained by coincidence.

The reconstruction helped establish the comparative method during the nineteenth century. Linguists have proposed vocabulary and grammatical forms for PIE, including terms associated with family relationships, animals, agriculture, and social organization. Some reconstructed forms are written with an asterisk to show that they are hypotheses rather than directly attested words.

PIE is the proposed ancestor of branches such as Indo-Iranian, Greek, Italic, Germanic, Celtic, Balto-Slavic, Armenian, Albanian, Anatolian, and Tocharian. Its homeland and exact date remain debated; the Pontic-Caspian steppe is prominent in current scholarship, but the language itself remains a reconstruction, not a known ancient inscriptional language.