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 go 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.
The number of words in a language is more than the words listed in one dictionary. One dictionary may have a list of 500,000 (half a million) words. Another dictionary may have some other words that the other dictionary does not have. When you add all the words in those dictionaries, there are about 750,000 words in English. There may be more words than that.
You may think to yourself, "If there are 750,000 words, how can we talk with only 3000 words?" Because, we do not need to know all the words. You can say most things with around 3,000 words.
The vocabulary of a language is always changing. New words are made or words change their meaning. Words about computers, like "download" are new to the English language. The new word "bling" came from hip hop. Words like "cool" have developed new meanings.
Definition and usage
Vocabulary is commonly defined as "all the words known and used by a particular person".
Productive and receptive knowledge
The first major change distinction that must be made when evaluating word knowledge is whether the knowledge is productive (also called achieve) or receptive (also called receive); even within those opposing categories, there is often no clear distinction. Words that are generally understood when heard or read or seen constitute a person's receptive vocabulary. These words may range from well-known to barely known (see degree of knowledge below). A person's receptive vocabulary is usually the larger of the two. For example, although a young child may not yet be able to speak, write, or sign, he or she may be able to follow simple commands and appear to understand a good portion of the language to which they are exposed. In this case, the child's receptive vocabulary is likely tens, if not hundreds of words, but his or her active vocabulary is zero. When that child learns to speak or sign, however, the child's active vocabulary begins to increase. It is also possible for the productive vocabulary to be larger than the receptive vocabulary, for example in a second-language learner who has learned words through study rather than exposure, and can produce them, but has difficulty recognizing them in conversation.
Productive vocabulary, therefore, generally refers to words that can be produced within an appropriate context and match the intended meaning of the speaker or signer. As with receptive vocabulary, however, there are many degrees at which a particular word may be considered part of an active vocabulary. Knowing how to pronounce, sign, or write a word does not necessarily mean that the word that has been used correctly or accurately reflects the intended message; but it does reflect a minimal amount of productive knowledge.
Degree of knowledge
Within the receptive–productive distinction lies a range of abilities that are often referred to as degree of knowledge. This simply indicates that a word gradually enters a person's vocabulary over a period of time as more aspects of word knowledge are learnt. Roughly, these stages could be described as:
- Nation, Paul, and Robert Waring. 1997. Vocabulary size, text coverage and word lists. Vocabulary: Description, acquisition and pedagogy, 6-19. 
- Goulden R; Nation P. & Read J. 1990. How large can a receptive vocabulary be? Applied Linguistics 11: 341-363.
- Crystal, David 1995. The Cambridge encyclopedia of the English language. Cambridge University Press, p119, p423.
- Adolphs and Schmitt 2003. Lexical coverage of spoken discourse
- Zipf G.K. 1949. Human behavior and the principle of least effort. Cambridge, Massachusetts: Addison-Wesley.
- Cambridge Advanced Learners Dictionary
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