The Utterance or Word
In Bakhtin's view, an expression in a living context of exchange--termed a "word" or "utterance"--is the main unit of meaning (not abstract sentences out of context), and is formed through a speaker's relation to Otherness (other people, others' words and expressions, and the lived cultural world in time and place). A "word" is therefore always already embedded in a history of expressions by others in a chain of ongoing cultural and political moments.
An utterance/word is marked by what Bakhtin terms "Addressivity" and "Answerability" (it is always addressed to someone and anticipates, can generate, a response, anticipates an answer). Discourse (chains or strings of utterances) is thus fundamentally dialogic and historically contingent (positioned within, and inseparable from, a community, a history, a place).
"I live in a world of others' words." (Problems of Dostoevsky's Poetics, 143)
"Any understanding of live speech, a live utterance, is inherently responsive... Any utterance is a link in the chain of communication." (Speech Genres, 68, 84)
"The word lives, as it were, on the boundary between its own context and another, alien, context." (Dialogic Imagination, 284).
Heteroglossia and Polyphony
Speech and complex cultural discourse in all our genres (novels, scientific descriptions, art works, philosophical arguments, for example) is mixed through and through with heteroglossia (an other's speech, and many others' words, appropriated expressions) and are necessarily polyphonic ("many-voiced," incorporating many voices, styles, references, and assumptions not a speaker's "own").
Every level of expression from live conversational dialog to complex cultural expression in other genres and art works is an ongoing chain or network of statements and responses, repetitions and quotations, in which new statements presuppose earlier statements and anticipate future responses.