In a language or dialect, a phoneme (from the Greek: φώνημα, phōnēma, "a sound uttered") is the smallest segmental unit of sound employed to form meaningful contrasts between utterances
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.<br />
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Linguists sometimes use utterance to simply refer to a unit of speech under study. The corresponding unit in written language is text.