isogloss

plural isoglosses

1. human sciencesA 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, The Calcutta Review, Calcutta: University of Calcutta, →OCLC, page 285

   1 more example

   The convergence of multiple isoglosses on a map, a "bundle", indicates a boundary between one regional dialect and another.

   2005, Brian D. Joseph, Carol G. Preston, Dennis R. Preston, editors, Language Diversity in Michigan and Ohio: Towards Two State Linguistic Profiles, Ann Arbor, Mich.: Caravan Books, →ISBN, page 29

   Synonyms: heterogloss

2. human sciencesA 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 December 12, Tom Hercules Davies, “Horses, Wheels, Languages: Greek, Mesopotamian and Indic Epic on the Eurasian Steppe”, in The Classical Quarterly, volume 75, number 1, page 13

   2 more examples

   There was also a tendency to treat transitive verbs passively, as in Hurrian and Urarṭian; it must, however, be emphasized that this phenomenon may also be considered as an isogloss rather than as an indication of genetic relationship.

   1970, W[illiam] F[oxwell] Albright, T[homas] O[den] Lambdin, “The Evidence of Language”, in I[orwerth] E[iddon] S[tephen] Edwards, C[yril] J[ohn] Gadd, N[icholas] G[eoffrey] L[emprière] Hammond, editors, The Cambridge Ancient History, 3rd edition, volumes I, part 1 (Prolegomena and Prehistory), Cambridge: Cambridge University Press, published 2000, →ISBN, section IV (Sumerian, Hurrian, Urarṭian, Elamite), page 154

   It is likely that the Sardinian forms are related to Lat. afrissa 'dragon lily'. The question regarding its etymology is all but resolved however. It is not quite clear where Pseudo-Apuleius' herbarium [the only known use of afrissa] was composed. If it is North African, as has been proposed, this might be yet another instance of a Sardinian-African Romance isogloss, possibly due to Punic influence.

   2025, Cid Swanenvleugel, The Pre-Roman Elements of the Sardinian Lexicon, page 66