Vernacular is the ordinary, informal, spoken form of language, particularly when perceived as having lower social status or less prestige than standard language, which is more codified, institutionally promoted, literary, or formal. More narrowly, a particular language variety that does not hold a widespread high-status perception, and sometimes even carries social stigma, is also called a vernacular, vernacular dialect, nonstandard dialect, etc. and is typically its speakers' native variety. Regardless of any such stigma, all nonstandard dialects are full-fledged varieties of language with their own consistent grammatical structure, sound system, body of vocabulary, etc.

Like any native language variety, a vernacular has an internally coherent system of grammar. It may be associated with a particular set of vocabulary, and spoken using a variety of accents, styles, and registers. As American linguist John McWhorter describes about a number of dialects spoken in the American South in earlier U.S. history, including older African-American Vernacular English, "the often nonstandard speech of Southern white planters, nonstandard British dialects of indentured servants, and West Indian patois, [...] were nonstandard but not substandard." In other words, the adjective "nonstandard" should not be taken to mean that these various dialects were intrinsically incorrect, less logical, or otherwise inferior, only that they were not the socially perceived norm or mainstream considered prestigious or appropriate for public speech; however, nonstandard dialects are indeed often stigmatized as such, due to socially-induced post-hoc rationalization. Again, however, linguistics regards all varieties of a language as coherent, complex, and complete systems—even nonstandard varieties.

A dialect or language variety that is a vernacular may not have historically benefited from the institutional support or sanction that a standard dialect has. According to another definition, a vernacular is a language that has not developed a standard variety, undergone codification, or established a literary tradition.

Vernacular may vary from overtly prestigious speech varieties in different ways, in that the vernacular can be a distinct stylistic register, a regional dialect, a sociolect, or an independent language. Vernacular is a term for a type of speech variety, generally used to refer to a local language or dialect, as distinct from what is seen as a standard language. The vernacular is contrasted with higher-prestige forms of language, such as national, literary, liturgical or scientific idiom, or a lingua franca, used to facilitate communication across a large area. However, vernaculars usually carry covert prestige among their native speakers, in showcasing group identity or sub-culture affiliation.

As a border case, a nonstandard dialect may even have its own written form, though it could then be assumed that the orthography is unstable, inconsistent, or unsanctioned by powerful institutions, like that of government or education. The most salient instance of nonstandard dialects in writing would likely be nonstandard phonemic spelling of reported speech in literature or poetry (e.g., the publications of Jamaican poet Linton Kwesi Johnson) where it is sometimes described as eye dialect.

Nonstandard dialects have been used in classic literature throughout history. One famous example of this is Mark Twain, The Adventures of Huckleberry Finn. This classic piece of literature, commonly taught in schools in the U.S., includes dialogue from various characters in their own native vernaculars (including representations of Older Southern American English and African-American English), which are not written in standard English.

In the case of the English language, while it has become common thought to assume that nonstandard varieties should not be taught, there has been evidence to prove that teaching nonstandard dialects in the classroom can encourage some children to learn English.

The first known usage of the word "vernacular" in English is not recent. In 1688, James Howell wrote: