isogloss

noun2

1. •
   A line on a map indicating the geographical boundaries of a linguistic feature.

   • Bloch thinks (and we agree with him) that details of isogloss will probably not bear out this kind of grouping. — 1921
2. •
   A linguistic feature shared between languages or language varieties, through any of various mechanisms, whether genetic inheritance or other linguistic forces.

   • PIE and its earliest-attested descendants evince a number of isoglosses with the Semitic languages of West Asia, indicating borrowing in one direction or another. — 2025