Isogloss

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An isogloss is a geographical border between a particular type of linguistic features.

(The name seems inspired by isolines, contour lines, and yes, that makes this use incorrect - those join identical features rather than separate significantly different ones)

Isoglosses mark the places with a comparatively sharp contrast in a particular aspect of pronunication, gloss meaning, or such.

When isoglosses bundle up they are used to mark the borders between related dialects (e.g. the Benrath line) or even languages (somewhat roughly) as with the and La Spezia-Rimini line.