For example, most of the various regional Romance languages of Italy, often colloquially referred to as Italian "dialects", are, in fact, not actually derived from modern standard Italian, but rather evolved from Vulgar Latin separately and individually from one another and independently of standard Italian, long prior to the diffusion of a national standardized language throughout what is now Italy. These various Latin-derived regional languages are, therefore, in a linguistic sense, not truly "dialects" or varieties of the standard Italian language, but are instead better defined as their own separate languages. Conversely, with the spread of standard Italian throughout Italy in the 20th century, regional versions or varieties of standard Italian have developed, generally as a mix of national standard Italian with a substratum of local regional languages and local accents. While "dialect" levelling has increased the number of standard Italian speakers and decreased the number of speakers of other languages native to Italy, Italians in different regions have developed variations of standard Italian particular to their region. These variations on standard Italian, known as regional Italian, would thus more appropriately be called "dialects" in accordance with the first linguistic definition of "dialect", as they are in fact derived partially or mostly from standard Italian.
A dialect is distinguished by its vocabulary, grammar, and pronunciation (phonology, including prosody). Where a distinction can be made only in terms of pronunciation (including prosody, or just prosody itself), the term accent may be preferred over dialect. Other types of speech varieties include jargons, which are characterized by differences in lexicon (vocabulary); slang; patois; pidgins; and argots. The particular speech patterns used by an individual are termed an idiolect.
A standard dialect (also known as a standardized dialect or "standard language") is a dialect that is supported by institutions. Such institutional support may include government recognition or designation; presentation as being the "correct" form of a language in schools; published grammars, dictionaries, and textbooks that set forth a correct spoken and written form; and an extensive formal literature that employs that dialect (prose, poetry, non-fiction, etc.). There may be multiple standard dialects associated with a single language. For example, Standard American English, Standard British English, Standard Canadian English, Standard Indian English, Standard Australian English, and Standard Philippine English may all be said to be standard dialects of the English language.
A nonstandard dialect, like a standard dialect, has a complete vocabulary, grammar, and syntax, but is usually not the beneficiary of institutional support. Examples of a nonstandard English dialect are Southern American English, Western Australian English, Scouse and Tyke. The Dialect Test was designed by Joseph Wright to compare different English dialects with each other.
There is no universally accepted criterion for distinguishing two different languages from two dialects (i.e. varieties) of the same language. A number of rough measures exist, sometimes leading to contradictory results. The distinction is therefore subjective and depends upon the user's frame of reference. For example, there has been discussion about whether or not the Limón Creole English should be considered "a kind" of English or a different language. This creole is spoken in the Caribbean coast of Costa Rica (Central America) by descendants of Jamaican people. The position that Costa Rican linguists support depends upon which University they represent.
The most common, and most purely linguistic, criterion is that of mutual intelligibility: two varieties are said to be dialects of the same language if being a speaker of one variety confers sufficient knowledge to understand and be understood by a speaker of the other; otherwise, they are said to be different languages. However, this definition becomes problematic in the case of dialect continua, in which it may be the case that dialect B is mutually intelligible with both dialect A and dialect C but dialects A and C are not mutually intelligible with each other. In this case, the criterion of mutual intelligibility makes it impossible to decide whether A and C are dialects of the same language or not. The mutual intelligibility criterion also flounders in cases in which a speaker of dialect X can understand a speaker of dialect Y, but not vice versa.
Another occasionally used criterion for discriminating dialects from languages is the sociolinguistic notion of linguistic authority. According to this definition, two varieties are considered dialects of the same language if (under at least some circumstances) they would defer to the same authority regarding some questions about their language. For instance, to learn the name of a new invention, or an obscure foreign species of plant, speakers of Westphalian and East Franconian German might each consult a German dictionary or ask a German-speaking expert in the subject. Thus these varieties are said to be dependent on, or heteronomous with respect to, Standard German, which is said to be autonomous. In contrast, speakers in the Netherlands of Low Saxon varieties similar to Westphalian would instead consult a dictionary of Standard Dutch. Similarly, although Yiddish is classified by linguists as a language in the Middle High German group of languages, a Yiddish speaker would consult a different dictionary in such a case.