Wikipedia: Dialect

The term dialect is used in linguistics to distinguish old vernacular languages from informal language derived from the standard language (called "Umgangssprache" in German) and the standard language itself. Insofar, the term has a special function in linguistics, although everyday usage may not be accurate from the dialectologist's point of view. In many countries dialects pretty much died out, e.g. in France, where many dialects and languages like Occitan (once a literary language) were spoken.

In a way, dialect is not a linguistic term: there is no means by which one is able to distinguish a dialect from a language - it is rather a kind of sociolinguistic evaluation (or historical evaluation in the case of linguists) that makes people talk about languages and dialects. Often, dialects are solely called dialects because they are NOT (or not recognized as) literary languages, because they are not standardized, because the speakers of the given language do not have a state of their own, because their language lacks prestige etc.

"La linguistica odierna sa che lo statuto di lingua non viene determinato soltanto da criteri linguistici, bensì è anche il risultato di uno sviluppo storico e politico. Il romancio si è imposto in diversi campi come lingua scritta, diventando così, malgrado una parentela a volte stretta con i dialetti lombardi alpini, una lingua a sé stante." (G 13) Gross, Manfred, Bernard Cathomas e Jean-Jacques Furer. 1996. RETOROMANCIO Facts & Figures Coira: Lia Rumantscha. (=G)

(Modern day linguistics knows that the status of language is not solely determined by linguistic criteria, but it is also the result of a historical and political development. Romansh came to be a written language, and therefore it is recognized as a language, even though it is very close to the Lombardic alpine dialects.)