Heteroglossia is the coexistence of distinct linguistic varieties, styles of discourse, or points of view within a single language (in Greek: hetero- "different" and glōssa "tongue, language"). The term translates the Russian разноречие [raznorechie: literally, "varied-speechedness"], which was introduced by the Russian literary theorist Mikhail Bakhtin in his 1934 paper Слово в романе [Slovo v romane], published in English as "Discourse in the Novel." The essay was published in English in the book The Dialogic Imagination: Four Essays by M. M. Bakhtin, translated and edited by Michael Holquist and Caryl Emerson.

Heteroglossia is the presence in language of a variety of "points of view on the world, forms for conceptualizing the world in words, specific world views, each characterized by its own objects, meanings and values." For Bakhtin, this diversity of "languages" within a single language brings into question the basic assumptions of system-based linguistics. Every word uttered, in any specific time or place, is a function of a complex convergence of forces and conditions that are unique to that time and place. Heteroglossia is thus "the base condition governing the operation of meaning in any utterance" and that which always guarantees "the primacy of context over text." It is an attempt to conceptualize the reality of living discourse, where there is always a tension between centralizing and decentralizing forces. According to Bakhtin, linguistics—to the extent that it operates on the presumption that language is a system—inevitably suppresses the fundamentally heteroglot nature of language as it is lived and experienced by human beings in their day to day realities.

Any language, in Bakhtin's view, stratifies into many voices: "social dialects, characteristic group behaviour, professional jargons, generic languages, languages of generations and age groups, tendentious languages, languages of the authorities, of various circles and of passing fashions". This diversity of voice is, Bakhtin asserts, the defining characteristic of the novel as a genre. When heteroglossia is incorporated into the novel, it is "another's speech in another's language, serving to express authorial intentions but in a refracted way".

Bakhtin proposes that these stratifications of language represent distinct points of view on the world, characterized by their own meanings and values. In this view, language is "shot through with intentions and accents", and thus there are no neutral words. Even the most unremarkable statement possesses a taste, whether of a profession, a party, a generation, a place or a time. To Bakhtin, words do not exist until they are spoken, and in that moment they are imprinted with the signature of the speaker.

Bakhtin identifies the act of speech, or of writing, as a literary-verbal performance, one that requires speakers or authors to take a position, even if only by choosing the dialect in which they will speak. Separate languages are often identified with separate circumstances. The prose writer, Bakhtin argues, must welcome and incorporate these many languages into their work.

Bakhtin rejects the idea that language is a system of abstract norms and that the utterance is a mere instantiation of the system of language. In "Discourse in the Novel", he criticizes linguistics, poetics, and stylistics for misunderstanding the fact that different people and groups speak differently. According to Bakhtin, language, like the psyche and everything else in culture, is never a finished, ordered system: it is a work in progress, always ongoing, never complete. There is a constant tension in language between the attempt to impose order and the fact that life itself is essentially chaotic. Real life is complex, spontaneous, subjective, impulsive, not pre-determined, full of disorder, the unexpected, the unknown, the undefined, the indefinable, and it refuses to be (or rather it cannot be) contained in a system that imagines and imposes an order of things. These dis-ordering forces in language, which Bakhtin refers to as centrifugal, are not unified or somehow conscious of themselves as forces of opposition. Centrifugal forces are essentially disparate and disunified: attempts to unify them are an ordering project, and thus not centrifugal.

The force in culture that strives for unity and order Bakhtin refers to as centripetal. It is reflected in language in the standardisation of national languages, in rules of grammar, the writing of dictionaries, and in the science of linguistics. Bakhtin does not object to such an effort, but he insists that it must be recognized as an imposition of order on something that fundamentally lacks it: "A unitary language is not something given [дан, dan] but is always in essence posited [задан, zadan]". Disciplines like philology, linguistics, stylistics and poetics take something that is an ideal, something that is posited in a struggle for social unity, and mistake it for something that really exists. The posited system is reified and an explanatory force is arbitrarily bestowed upon it, effectively denying the existence of the living, disordered, heteroglot reality upon which it is imposed. The attempt to systematize language—to objectify, idealize and abstract it into a static set of rules and conventions for signification—is falsely posited as a descriptive or scientific activity, when in reality it is a form of socio-political activism.

According to Bakhtin, language is always a multiplicity of languages. This is not merely a matter of dialectology, but of the many different ways of speaking, which are reflections of the diversity of social experience, of differing ways of conceptualizing and evaluating. Linguistics fails to appreciate the importance of this multiplicity in the reality of language as it is actually lived and practiced. It is not merely a matter of different vocabularies, but a complex of experiences, shared evaluations, ideas, perspectives and attitudes that are "knitted together" (срастаться, srastat'sya) in an organic process: a coalescence of separate entities that have themselves been formed by such a process, which is to say by a living process of adaptation and growth. Different languages reflect different attitudes and worldviews. Linguistic features are not fixed and definitive: they are a consequence—"traces", "crystallizations", or "sclerotic deposits"―of these attitudes and worldviews, which are themselves the consequence of particular forms of active participation in life and culture. Such participation is a creative response to the circumstances and demands of daily life: "discourse lives, as it were, beyond itself, in a living impulse (направленность, napravlennost') toward the object; if we wholly detach ourselves from this impulse all we have left is the naked corpse of the word, from which we can learn nothing at all about the social situation or the fate of a given word in life."