The term dialect (from Latin dialectus, dialectos, from the Ancient Greek word διάλεκτος, diálektos 'discourse', from διά, diá 'through' and λέγω, légō 'I speak') can refer to either of two distinctly different types of linguistic phenomena:

One usage refers to a variety of a language that is a characteristic of a particular group of the language's speakers. Under this definition, the dialects or varieties of a particular language are closely related and, despite their differences, are most often largely mutually intelligible, especially if geographically close to one another in a dialect continuum. The term is applied most often to regional speech patterns, but a dialect may also be defined by other factors, such as social class or ethnicity. A dialect that is associated with a particular social class can be termed a sociolect, a dialect that is associated with a particular ethnic group can be termed an ethnolect, and a geographical/regional dialect may be termed a regiolect (alternative terms include 'regionalect', 'geolect', and 'topolect'). According to this definition, any variety of a given language can be classified as a "dialect", including any standardized varieties. In this case, the distinction between the "standard language" (i.e. the "standard" dialect of a particular language) and the "nonstandard" (vernacular) dialects of the same language is often arbitrary and based on social, political, cultural, or historical considerations or prevalence and prominence. In a similar way, the definitions of the terms "language" and "dialect" may overlap and are often subject to debate, with the differentiation between the two classifications often grounded in arbitrary or sociopolitical motives. The term "dialect" is however sometimes restricted to mean "non-standard variety", particularly in non-specialist settings and non-English linguistic traditions.

The other usage of the term "dialect", specific to colloquial settings in a few countries like Italy (see dialetto), France (see patois), much of East Central Europe, and the Philippines, carries a pejorative undertone and underlines the politically and socially subordinated status of a non-national language to the country's single official language. In this case, these "dialects" are not actual dialects in the same sense as in the first usage, as they do not derive from the politically dominant language and are therefore not one of its varieties, but they evolved in a separate and parallel way and may thus better fit various parties' criteria for a separate language. These "dialects" may be historically cognate with and share genetic roots in the same subfamily as the dominant national language and may even, to a varying degree, share some mutual intelligibility with the latter. However, in this sense, unlike in the first usage, these "dialects" may be better defined as separate languages from the standard or national language and the standard or national language would not itself be considered a "dialect", as it is the dominant language in a particular state, be it in terms of linguistic prestige, social or political (e.g. official) status, predominance or prevalence, or all of the above. The term "dialect" used this way implies a political connotation, being mostly used to refer to low-prestige languages (regardless of their actual degree of distance from the national language), languages lacking institutional support, or those perceived as "unsuitable for writing". The designation "dialect" is also used popularly to refer to the unwritten or non-codified languages of developing countries or isolated areas, where the term "vernacular language" would be preferred by linguists.

Features that distinguish dialects from each other can be found in lexicon (vocabulary) and grammar, as well as in pronunciation (phonology, including prosody). Where the salient distinctions are only or mostly to be observed in pronunciation, the more specific term accent may be used instead of dialect. Differences that are largely concentrated in lexicon may be creoles in their own right. When lexical differences are mostly concentrated in the specialized vocabulary of a profession or other organization, they are jargons; differences in vocabulary that are deliberately cultivated to exclude outsiders or to serve as shibboleths are known as cryptolects (or "cant") and include slangs and argots. The particular speech patterns used by an individual are referred to as that person's idiolect.

To classify subsets of language as dialects, linguists take into account linguistic distance. The dialects of a language with a writing system will operate at different degrees of distance from the standardized written form. Some dialects of a language are not mutually intelligible in spoken form, leading to debate as to whether they are regiolects or separate languages.