Noun
isogloss (plural isoglosses)
(linguistics) A line on a map indicating the geographical boundaries of a linguistic feature.
Synonym: heterogloss
A. F. Gupta's survey of mostly middle-class students found that /æ/ was used by almost everyone who was from clearly north of the isogloss for BATH words. Source: Internet
This isogloss is considered as the most imperative within all Swedish pronunciation differences. Source: Internet