Broadly, two criteria define a language and distinguish it from a dialect: political and social.
In the political view, languages are ‘prestigious’, are both spoken & written and should have grammar, vocabulary and syntax (ability of words to come together to create well-formed sentences) to enable it to be used as “official”.
Dialects, on the other hand, are only spoken; they are unofficial and thus are not hugely respected.
Because languages are prestigious and dialects are not, no country would want to ‘devalue’ its language by calling it a dialect of some other language, no matter how close that ‘language’ is to the language of the neighbouring country.
For example, people of Norway, Sweden & Denmark can comfortably understand each other but none would call their language a dialect of the other.
That is what brings us to the second criterion: understanding, the social aspect of defining a language.
In the social view, if two related kinds of speech are so close that the speakers can understand ‘each other’ (without having learnt it as a foreign language), they are dialects of the same language.
“Each Other” is very important e.g. people of Netherlands can understand German pretty well but people of Germany can’t understand Dutch (language of Netherlands), so they are different languages.
Having said that, the social aspect of language has always been very tricky and that is where micro-criteria for language come in.
Micro-criterion 1: It should have vitality; i.e. it should be used extensively both inside and outside homes, by all generations, and for most, if not all topics.
Micro-crietrion 2: It should have autonomy i.e. it should feel different from other languages, but this criterion can be subjective.
Micro-criterion 3: It should have norms or rules i.e. you should be able to identify good speakers from the bad speakers; this is an offshoot of a language having a grammar, syntax and vocabulary because these define whether someone is a good speaker or a bad speaker of the language.
Micro-criteria can go on and on and that is what makes “language Vs. dialect” a grey area, which has also led to many controversies in history.
