Underextension

Underextension occurs when a categorical term (a word used to describe a group of things) is used in language improperly by only using it for one object instead of all objects that belong in that category. This often occurs in children when they are initially acquiring and developing language. Children will frequently not apply a new word to objects that it should be applied to.

An example would be a toddler learning the word ball to describe their toy ball. Underextension occurs when the child uses the word ball in reference to only their specific individual ball and not use it as a term for all balls. Another example would be a child using the word cat for only their family's lone pet cat instead of using it to apply to all cats.

Underextension declines as children develop and refine their vocabulary.