Mean length of utterance (MLU) refers to a measure of language development defined by the average number of meaningful language units (root words and endings) a child uses at any one time. Mean length of utterance (MLU), moreover, is defined as the common measure of grammatical development. It is the average length of the utterances in a sample of spontaneous speech, often counted in terms of the number of morphemes.
Lexicon of Psychology - Terms, Treatments, Biographies,