Some of the twenty volumes of the printed version of the OED (Oxford English Dictionary). Today, the online and CD versions are more used.

A vocabulary is a list of words.

The vocabulary a person uses is all the words that person knows and uses. In general, a person who is five knows about 4,000 to 5,000 words.Adults who have gone to college may know 20,000 words. A hearing vocabulary and reading vocabulary are bigger than a speaking vocabulary or writing vocabulary, as people understand some words that they do not use.