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 - isolines join identical values/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.

Isoglosses can also be 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.

(Isoglosses are often based on finding some local contrasts, this is often based on a collection of such isogloss contrasts, though in dialectology such analyses may try a large amount of localized samples to map this more gradually, see e.g. maps that L04 has made)