Words are single units of meaningful speech or writing, and are made up of letters and syllables, and even punctuation marks, like the apostrophe in Peter’s. They can be nouns, adjectives, verbs, names of people, cities, toys, brands, abbreviations and acronyms, and even sounds (onomatopoeia), like the following (regular) verb - The cows didn’t stop mooing all day!
They are used to describe ideas, actions, things, people, colours – anything we need to talk about or think about.
 Words in the dictionary
For number of words in English see main article Number of words in English.
A word exists if people use it, so the notion that a word can only exist if it is "in the dictionary" is incorrect. It only means that a particular dictionary at a particular time has not included that particular word for whatever reason. Which brings us rather inconveniently to the matter of just how many words there are in the English language, or how many words there are in the dictionary or...
At least in the Western hemisphere, brands like Mercedes, Ford, Coca-Cola or Sony form part of almost every language – not just English. Words which originally start out as acronyms, like radar and sonar, become incorporated as “normal” words very quickly.
One of the advantages of the flexibility of English is that it incorporates new words easily and can adapt them as necessary, depending on usage: Fax = noun and verb; Text = noun and verb; Texting = send text messages (SMS) using mobile phones; Google = verb, as in using the Google search engine to find information on the World Wide Web.
Over 80% of all words in English have two or more meanings, often, but not always, related in some way. So identifying the function of a word in its context is very important:
- Book can be a noun or a verb:
- I’ve read that book – I’d like to book a table for four;
- Water can be a noun or a verb:
- I want a drink of water - I must remember to water the plants;
- Drive can be a verb or a noun:
- Can you drive? – Did you have a nice drive?;
 How many words do I know?
Based on several studies, it would seem reasonable to estimate that an “average” person with an “average” education knows an average of 50,000 words either well or vaguely.
Estimates of passive vocabulary add another 25 per cent to a person’s lexicon. These averages exclude specialized terminology related to specific professions such as Medicine, Finance or Law, unless in general use, as well as many other fields such as sports, photography, food and cooking, etc., which contain many words common to the people interested in such matters, but unknown to others. Note also the significant difference between active and passive knowledge. Based on the above figures, a reasonably well-educated person probably uses around 12 per cent of the word stock of a language.
Furthermore, other studies suggest that the “average” person learns around 1,000 new words a year in his or her mother tongue until the age of 20. Foreign language students following a standard language coursebook can consider themselves lucky if they learn an average of 300 new words a year.
Recent studies show that the brain is not usually able to cope with learning more than 8 to 10 new words in one go. This, however, should not be considered any limitation whatsoever: 365 days x 8 words/day = 2,920 new words a year – not a bad lexicon to have in another language!).
Regardless of the number of words we may know, there is a general tendency, even in our mother tongue, to use fewer words in everyday speech. According to a study of native English speakers carried out in the early 80s these speakers only used around 600 different words in a normal day’s social conversation, excluding work-related matters. However, twenty years later, in the early years of the new millennium, the same research team found that the figure had dropped to 400 words.
And then, of course, there are the words we may know in a another language...
 Another view
The webiste Test your vocab runs a test for both native speakers and foreign learners. Based on their results they have established that "most Native English adult speakers who have taken the test fall in the range 20,000–35,000 words".
They have also produced some interesting statistics on vocabulary size of English students by country. this seems to give some impartial support to the frequently-heard claim that Scandinavians are good at English.
 How do I know I know a word?
And what precisely do we mean when we talk about “knowing” a word? First of all, we should consider some of the aspects involved, some more obvious than others. Ideally we should be able do do some, and ideally all, of the following:
- i. understand it when we come across it, whether spoken or in writing;
- ii. remember it when we need it;
- iii. use it with its correct meaning;
- iv. use it correctly from a grammatical point of view;
- v. pronounce it correctly;
- vi. know what other words can be used with it, that is, a fast car but NOT a cloudy car;
- vii. know how to spell it correctly;
- viii. use it in the right situation - its register;
- ix. know if it has negative or positive associations.
We must remember that each word, be it a verb, a noun, an adjective or whatever its function, can and must be able to relate to certain other words. That is, each word acts as a magnet to attract other words, other associations, other contexts.
Thus, when we learn the word car, we can associate it with at least some of the following variations: a fast car (+ adjective); drive a car (+ verb); car park; a car manufacturer (+ noun); I’ve got a car (sentence); a secondhand car dealer (as a phrase) in such a way as to not only situate the “new” word in a natural context but also to review other known words, thereby maintaining them fresh in our memory. From there, we can go on to closely-related words and expressions such as driver, passenger, motorist, mechanic, pedestrian, backseat driveretc.
Likewise, certain verbs require certain prepositions – depend on, listen to, try to – and that certain words require certain special uses: come/go/leave home but be/stay at home.
 Words and concepts
A constant debate is whether concepts such as facts – the names of people or places and other proper names be considered as forming part of one’s personal lexicon when calculating its size. Undoubtedly, the name, or fact, Shakespeare, is as much a part of the English language as the word literature or drama. And the fact/word London is probably used more often than the word village or town. Thus, given the overlapping of criteria, calculating the size of one’s own vocabulary is complicated and must vary according to many different factors.
- Trask, Larry "What is a word?"
- David Crystal's Blog
- Merriam-Webster online dictionary
- Test your vocabulary
- English vocabulary results by country.