isogloss

noun

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.”

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

2. A linguistic feature shared between languages or language varieties, through any of various mechanisms, whether genetic inheritance or other linguistic forces.

   “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.”

   “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.”