Toddlers make categorical overextensions when they extend a word they know to other words in the same category. For example, if a child learns the word dog and then calls all four-legged animals "dog," he or she is making a categorical overextension.
Overextension basically means the overly broad use of a word to the fact that its meaning is overgeneralized. "Doggie" is used in a broader context than is appropriate because all animals cannot apply to the criteria.
