In linguistics, two or more languages or dialects are said to be mutually unintelligible when a knowledge or familiarity of one language does not preclude a knowledge or familiarity of the other(s). Broadly conceived, linguistics is the scientific study of human language, and a linguist is someone who engages in this study. ... A dialect (from the Greek word διάλεκÏοÏ, dialektos) is a variety of a language used by people from a particular geographic area. ...
In linguistics, mutual intelligibility is a property exhibited by two or more distinct languages when speakers of one or more of the languages can readily understand at least one or more of the other language(s) without intentional study or extraordinary effort.
Mutual intelligibility can be asymmetric between the languages, with speakers of one understanding more of the other than speakers of the other understand of the first.
Russian and Polish are largely not mutually intelligible but Ukrainian is mutually intelligible to some degree to both, being believed by many to be an intermediary form in the dialect continuum.
If two speech varieties are not mutually intelligible, then the speech varieties are different languages; if they are mutually intelligible but differ systematically from one another, then they are dialects of the same language.
There are problems with this definition, however, because many levels of mutual intelligibility exist, and linguists must decide at what level speech varieties should no longer be considered mutually intelligible.
In addition, chains of speech varieties exist in which adjacent speech varieties are mutually intelligible, but speech varieties farther apart in the chain are not.