In linguistics, code-switching or language alternation occurs when a speaker alternates between two or more languages, or language varieties, in the context of a single conversation or situation.^{[citation needed]} These alternations are generally intended to influence the relationship between the speakers, for example, suggesting that they may share identities based on similar linguistic histories.

Code-switching is different from plurilingualism in that plurilingualism refers to the ability of an individual to use multiple languages, while code-switching is the act of using multiple languages together. Multilinguals (speakers of more than one language) sometimes use elements of multiple languages when conversing with each other. Thus, code-switching is the use of more than one linguistic variety in a manner consistent with the syntax and phonology of each variety.

Code-switching may happen between sentences, sentence fragments, words, or individual morphemes (in synthetic languages). However, some linguists consider the borrowing of words or morphemes from another language to be different from other types of code-switching.

Code-switching can occur when there is a change in the environment in which one is speaking, or in the context of speaking a different language or switching the verbiage to match that of the audience. There are many ways in which code-switching is employed, such as when speakers are unable to express themselves adequately in a single language or to signal an attitude towards something. Several theories have been developed to explain the reasoning behind code-switching from sociological and linguistic perspectives.

The earliest known use of the term "code-switching" in print was published in 1953, in a chapter by Roman Jakobson in Results Conf. Anthropologists & Linguists, written with C. F. Voegelin, T. A. Sebeok, and C. Lévi-Strauss. He attributes the idea to linguist William Freeman Twaddell, inspired by "communication engineers". In the 1950s, many scholars considered code-switching to be a substandard use of language. Since the 1980s, however, most scholars have come to regard it as a normal, natural product of bilingual and multilingual language use.

In popular usage and in sociolinguistic study, the term code-switching is frequently used to refer to switching among dialects, styles or registers. This form of switching is practiced, for example, by speakers of African American Vernacular English as they move from less formal to more formal settings. Such shifts, when performed by public figures such as politicians, are sometimes criticized as signaling inauthenticity or insincerity.

The term "code-switching" is also used outside the field of linguistics. Informally, code-switching is sometimes used to refer to relatively stable informal mixtures of two languages, such as Spanglish, Taglish, or Hinglish. Some scholars of literature use the term to describe literary styles that include elements from more than one language, as in novels by Chinese-American, Anglo-Indian, or Latino writers.

As switching between languages is exceedingly common and takes many forms, we can recognize code-switching more often as sentence alternation. A sentence may begin in one language, and finish in another. Or phrases from both languages may succeed each other in apparently random order. Such behavior can be explained only by postulating a range of linguistic or social factors such as the following: