isogloss
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.
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.
Where it comes from
From iso- + gloss, ultimately from Ancient Greek ἴσος (ísos, “equal”) (possibly from Proto-Indo-European *wi- (“to separate”)) + γλῶσσα (glôssa, “tongue; language”) (ultimately from Proto-Indo-European *glōgʰs).
Adapted from the Wiktionary entry isogloss (CC BY-SA 4.0).