Mutual intelligibility describes the degree to which speakers of two different language varieties (or lects) – particularly monolingual speakers – can understand each other. To the extent that linguists are able to do so, it’s the main criteria for determining whether two language varieties (or lects) are distinct languages or dialects of the same language. There are three main factors that determine the intelligibility between languages:
- lexical similarity, or how much of the vocabulary is shared
- grammar, because even if the words are mostly the same, if they’re put together in very different ways it can be hard to understand
- phonology – if the phonologies are very different, that limits comprehension even if the above two factors are favourable
Of course, in reality there’s not a simple binary between languages that are mutually intelligible or not, but a very fuzzy line. Even between two monolingual speakers, it can depend on the topic, speakers’ willingness to communicate (someone cranky who can’t really be bothered is going to put very little effort into understanding a different lect), the speakers’ knowledge of more archaic or rare synonyms of words in more common use (because it could be the rarer synonym in use in the other lect), the speakers’ innate puzzle-solving ability (to “fill the gaps” between the bits they understand). Furthermore, often in the real world people will have some knowledge of other languages (particularly related ones), which can improve intelligibility beyond the point you would reach with strictly monolingual speakers. There can also be a difference between spoken vs written language, with one being easier than the other depending on the language pair.
Asymmetric intelligibility, where there is more comprehension in one direction than the other, is another relevant phenomenon. For example, speakers of Portuguese generally have an easier time understanding Spanish than vice versa, because of Spanish’s more straightforward phonology and because reportedly more of the inherited, shared vocabulary has fallen into disuse in Spanish than Portuguese.