The term code-mixing is variously defined in different subfields of linguistics. Many studies of morphology or syntax use the term as a synonym for code-switching, the alternating use by bilingual speakers of two or more different languages within a single utterance (e.g. Kachru 1978, Muysken 2000).
Studies in psycholinguistics similarly definition code-mixing as "the transition from using linguistic units (words, phrases, clauses, etc.) of one language to using those of another within a single sentence" (Sridhar and Sridhar 1980). In contrast to Muysken's treatment of code-switching and code-mixing as synonymous, however, Sridhar and Sridhar differentiate the two, arguing that code-switching has pragmatic or discourse-oriented functions that may be absent in code-mixing.
Some recent work in sociolinguistics builds on this functional differentiation. Alvarez (1998) argues that formal code-mixing or language alternation should be treated as distinct from code-switching, defined in pragmatic or discourse terms.