The concrete speech act — not the abstract sentence but the actual words spoken by a particular person to a particular listener in a particular context, carrying dialogic traces of all prior and anticipated utterances.
In Bakhtin's framework, the utterance is the real unit of communication, as opposed to the sentence (the abstract unit of linguistics). A sentence is a grammatical structure existing outside time and social context; an utterance is what someone actually says, saturated with the speaker's intention, the listener's anticipated response, the history of how these words have been used before, and the social situation that makes this particular speech act meaningful. Utterances are inherently dialogic: they respond to prior utterances and anticipate replies. They are also inherently evaluative: every utterance takes a position, even when it appears neutral. Bakhtin's insistence on the utterance as the primary object of analysis was a challenge to structuralist linguistics, which studied language as a system of abstract relations. He argued that language lives not in dictionaries or grammars but in utterances — in actual, historically located, socially embedded acts of speech.