Speakers differ/vary at the level of phonetics and/or phonology only; their grammar and vocabulary is wholly or largely the same
Accent
Can index a speaker's regional/geographic origin or social factors such as level and type of education, social class, attitude
Dialect
A sub-variety of a language that differs in more than just phonetics and/or phonology, e.g. at the level of morphology, syntax and the vocabulary
All languages have dialects
Social dialects (sociolects)
Variation due to factors such as social class, gender, education, age, ethnicity, etc.
Branch of linguistics studying social dialects: sociolinguistics
Regional dialects (regiolects)
Variation due to geographical/regional/territorial factors
Branch of linguistics studying regional dialects: dialectology
Continuous variation
1. Regional dialects are spoken in continuous areas and form a dialect continuum
2. Dialects of the Germanic, Romance and Slavic languages: several dialect continua
Discontinuous variation
Dialects are spoken in discontinuous areas, i.e. interrupted by an area in which a different language is spoken
Proximity
Regional dialects which are spoken in adjacent areas are mutually intelligible and structurally similar
Dialects on the outer edges of the dialectal continuum may not be mutually intelligible, but are linked by a chain of mutual intelligibility
Differences among dialects are cumulative: the greater the geographical separation, the greater the differences
From continuous to discrete variation
Communicative isolation and passage of time may lead to the transformation of continuous variation into discrete variation, i.e. regional dialects turn into languages
The former dialects of Latin have turned into separate Romance languages
Mutual intelligibility
Varieties which are mutually intelligible are dialects of the same language. Varieties which are not mutually intelligible are languages.
Dialects may not be mutually intelligible: the dialects of Italian; the dialects of Chinese
Languages may be mutually intelligible: Serbian, Croatian and Bosnian; Bulgarian and Macedonian; Swedish, Danish and Norwegian; Hindi (India) and Urdu (Pakistan and India)
There is no unanimously accepted threshold for mutual intelligibility
Intelligibility may be higher in one direction: speakers of Portuguese understand Spanish more than speakers of Spanish understand Portuguese; Danes understand Norwegians better than Norwegians understand Danes
Structural similarity
Similarity in the components of a language, i.e. phonology, morphology, syntax and vocabulary. Varieties which are structurally similar are dialects of the same language. Varieties which are structurally different are languages.
Dialects of a language may display significant structural differences: the dialects of Chinese; the dialects of Italian; the dialects of German
Languages may be structurally very similar: Serbian, Croatian and Bosnian; Bulgarian and Macedonian; Swedish, Danish and Norwegian; Hindi and Urdu
There is no unanimously accepted threshold for structural similarity
Dialects may display structural similarity in some domains, but significant structural differences in other domains
Non-linguistic criteria
Include, but are not limited to: number of speakers, tradition, prestige, national identity
Typical scenario for the emergence of a language
A dialect predominates and turns into the standard variety of a language. This standard variety is called "language".
Dialects start out as equals, but one of them ends up as the "language". Only this variety is standardized and used by authorities, in the educational system, etc.
A variety may be perceived by its speakers as a distinct language, no matter how similar it is structurally to some other variety
A variety may be perceived by its speakers as being the same language as that of a standard variety, no matter how structurally distinct those varieties are
Moldovan
Structurally quasi-identical with Romanian, but perceived as a separate language by Moldovan nationalists
Galician
Structurally similar to Portuguese, but perceived as a Spanish dialect, a dialect of Portuguese, or a separate language
Swedish-Danish dialect continuum
Until 1658 southern Sweden was part of Denmark; the dialects spoken in the area were considered to be dialects of Danish. After the territory became part of Sweden, in reportedly some 40 years or so, the same dialects were considered dialects of Swedish.
Dutch-German dialect continuum
The Dutch-German border sometimes runs between neighbouring villages or towns; people cross the border regularly, e.g. to commute to their jobs, to go shopping, etc.
Speakers of dialect d3 understand and are understood by speakers of d'1, even though they are spoken on different sides of the border; d3 and d'1 are structurally similar
Where Dutch ends and where German starts, and to which language dialects d3 and d'1 should be assigned, frequently depend on non-linguistic factors
Structural Similarity
A similarity in grammar, vocabulary, or syntax between dialects.
Structural Differences
A significant difference in phonology, morphology, syntax, vocabulary, or pragmatics between dialects.
Dialects
A form of a language that is specific to a particular region, group of people, or social class.