In sociolinguistics, a variety, also known as a lect or an isolect, is a specific form of a language or language cluster. This may include languages, dialects, registers, styles, or other forms of language, as well as a standard variety. The use of the word variety to refer to the different forms avoids the use of the term language, which many people associate only with the standard language, and the term dialect, which is often associated with non-standard language forms thought of as less prestigious or "proper" than the standard. Linguists speak of both standard and non-standard (vernacular) varieties as equally complex, valid, and full-fledged forms of language. Lect avoids the problem in ambiguous cases of deciding whether two varieties are distinct languages or dialects of a single language.

Variation at the level of the lexicon, such as slang and argot, is often considered in relation to particular styles or levels of formality (also called registers), but such uses are sometimes discussed as varieties as well.

O'Grady et al. define dialect: "A regional or social variety of a language characterized by its own phonological, syntactic, and lexical properties." A variety spoken in a particular region is called a regional dialect (regiolect, geolect); some regional varieties are called regionalects or topolects, especially to discuss varieties of Chinese. In addition, there are varieties associated with particular ethnic groups (sometimes called ethnolects), socioeconomic classes (sometimes called sociolects), or other social or cultural groups.

Dialectology is the study of dialects and their geographic or social distribution. Traditionally, dialectologists study the variety of language used within a particular speech community, a group of people who share a set of norms or conventions for language use.

In order to sidestep the vexing problem of distinguishing dialect from language, some linguists have been using the term communalect – defined as "a neutral term for any speech tradition tied to a specific community".

More recently, sociolinguists have adopted the concept of the community of practice, a group of people who develop shared knowledge and shared norms of interaction, as the social group within which dialects develop and change. Sociolinguists Penelope Eckert and Sally McConnell-Ginet explain: "Some communities of practice may develop more distinctive ways of speaking than others. Thus, it is within communities of practice that linguistic influence may spread within and among speech communities."

The words dialect and accent are often used synonymously in everyday speech, but linguists define the two terms differently. Accent generally refers to differences in pronunciation, especially those that are associated with geographic or social differences, whereas dialect refers to differences in grammar and vocabulary as well.

Many languages have a standard variety, some lect that is selected and promoted prescriptively by either quasi-legal authorities or other social institutions, such as schools or media. Standard varieties are accorded more sociolinguistic prestige than other, nonstandard lects and are generally thought of as "correct" by speakers of the language. Since the selection is an arbitrary standard, standard forms are the "correct" varieties only in the sense that they are tacitly valued by higher socio-economic strata and promoted by public influencers on matters of language use, such as writers, publishers, critics, language teachers, and self-appointed language guardians. As Ralph Harold Fasold puts it, "The standard language may not even be the best possible constellation of linguistic features available. It is general social acceptance that gives us a workable arbitrary standard, not any inherent superiority of the characteristics it specifies."