Use of a sound in a wider range than permitted by the adult language.  For example, a child might refer to a ‘tiger’ as a ‘kitty’ or ‘mouses’ for ‘mice’, in the latter case overgeneralization the morphological rule for plural nouns.  It is not a case of imitating adult speech, but rather attempts to work out grammatical rules involving plural nouns and past tenses (e.g., ‘bringed’ for ‘brought’).  Children eventually adjust their self-formulated rules in order to accommodate such exceptions in past tenses and plural nouns, something referred to as the ‘retreat from overgeneralization‘.  This process of change can appear to be U-shaped in that children seem to be getting worse in their accuracy of using irregular past tenses and plural nouns.  The opposite of overgeneralization is undergeneralization.        

See Language development, Pleonastic extensions, Undergeneralization