Final answer:
The mean length of utterance refers to the number of morphemes in a sentence, representing the average length of spoken sentences in terms of meaningful language units.
Step-by-step explanation:
The mean length of utterance (MLU) refers to the average length of a person's spoken sentences, measured in morphemes. Morphemes are the smallest units of language that convey meaning and can be words, prefixes, or suffixes. This measurement is often used in the study of language development to assess a person's linguistic growth over time. Unlike phonemes, which are the smallest units of sound in speech, or vowels, which are a type of speech sound characterized by a lack of closure in the vocal tract, morphemes are significant for their meaning rather than their sound alone. Thus, the mean length of utterance refers to the number of morphemes in a sentence.