Utterance does not have a precise linguistic definition. Phonetically an utterance is a unit of speech bounded by silence. In dialogue, each turn by a speaker may be considered an utterance.
Linguists sometimes use utterance to simply refer to a unit of speech under study. The corresponding unit in written language is text .
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Aronoff, Mark and Janie Rees-Miller (editors). 2001.The handbook of linguistics. Blackwell Handbooks in Linguistics. Oxford: Blackwell. xvi–823.