An isogloss is the geographical boundary of a certain linguistic feature, e.g. the pronunciation of a vowel, the meaning of a word, or use of some syntactic feature. Dialects are typically demarcated by whole bundles of isoglosses, e.g. the Benrath line that divides High German and Low German; or the La Spezia-Rimini Line which divides the eastern Romance languages from the western ones.
The name is inspired by contour lines such as isobar, etc.; however the isogloss separates, rather than connecting points of equal language (perhaps one can say it connects points of indefinite language).
