A word or group of words, normally in speech, that make sense by themselves but do not necessarily contain the grammatical requirements of sentences found in more formal written language. The Longman Dictionary of Language Teaching and Applied Linguistics (2010) says of an utterance: ‘a unit of analysis in speech which has been defined in various ways but most commonly as a sequence of words within a single person’s turn at talk that falls under a single intonation contour. Utterances may sometimes consist of more than one sentence, but more commonly consist of stretches of speech shorter than sentences’. The term utterance is often used in contrast to sentence in written language.
"This little dialogue contains two utterances:
A: He didn’t really understand what was going on.
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Richards, J. and Schmidt, R. (2010) Longman Dictionary of Language Teaching and Applied Linguistics. Harlow: Pearson.