Wiktionary
n. A line indicating the geographical boundaries of a linguistic feature on a map.
Wikipedia
An isogloss, also called a heterogloss (see Etymology below), is the geographic boundary of a certain linguistic feature, such as the pronunciation of a vowel, the meaning of a word, or the use of some syntactic feature. Major dialects are typically demarcated by groups of isoglosses such as the Benrath line distinguishes High German from the other West Germanic languages; and the La Spezia–Rimini Line divides the Northern Italian dialects from Central Italian dialects. However, an individual isogloss may or may not have any coincidence with a language border. For example, the front-rounding of /y/ cuts across France and Germany, while the /y/ is absent from Italian and Spanish words that are cognate with the /y/-containing French words.
One of the most well-known isoglosses is the centum-satem isogloss.
Similar to an isogloss, an isograph is a distinguishing feature of a writing system. Both concepts are also used in historical linguistics.
Usage examples of "isogloss".
Juju also told him that this is an isogloss, a region of distinct accents and dialects.
But it is perfectly possible to draw what linguists would call the isoglosses of a myth, the lines which limit the social region where it is spoken.