An isogloss refers to a specific type of language border. It's the geographical boundary or delineation of a certain linguistic feature, for example the pronunciation of a vowel, the meaning of a word, or use of some syntactic feature. Major dialects are typically demarcated by whole bundles of isoglosses, for example the Benrath line that distinguishes High German from the other West Germanic languages; or the La Spezia-Rimini Line which divides the Northern Italian dialects from Central ones.… (More on Isogloss)