A person's vocabulary's the set of words they're familiar with in a language. A vocabulary usually grows and evolves with age, and serves as a useful and fundamental tool for communication and acquiring knowledge. Knowing and using a word A vocabulary's defined as "all the words known and used by a particular person". However, the words known and used by a particular person don't constitute all the words a person's exposed to. By definition, a vocabulary includes the last two categories of th… (More on Vocabulary)