An isogloss—also called a heterogloss (see Etymology below)—is the geographical boundary of a certain linguistic feature, such as the pronunciation of a vowel, the meaning of a word, or use of some syntactic feature. Major dialects are typically demarcated by groups of isoglosses; for example 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.