Also known as underextension, it is a failure to use a word across its full adult range. For example, a child might restrict the word ‘dog’ by only allowing it to refer to the family pet. One of the explanations forundergeneraliztion is that children, especially those around two years-of-age, are not able to tell the difference betweeninherent properties of things and accidental ones. In the case of calling a dog a dog when it is the family pet.
See Language development, Overgeneralization, Pleonastic extensions