Mutual intelligibility - Wikipedia
https://en.wikipedia.org/wiki/Mutual_intelligibilitySome linguists use mutual intelligibility as a primary criterion for determining whether two speech varieties represent the same or different languages. In a similar vein, some claim that mutual intelligibility is, ideally at least, the primary criterion separating languages from dialects. A primary challenge to these positions is that speakers of closely related languages can often communicate with each other effectively if they choose to do so. In the case of transparently co…