In general terms, a Lexicon is a list of words. It is almost synonymous with vocabulary.
For example, we can talk about:
a personal lexicon – the words which an individual knows
a social class lexicon – the words used by a certain class of person; of course this can be extended to talk about the lexicon of any group of people
a lexicon of English – all the words in the English language
the Harry Potter lexicon – specialized words used in the Harry Potter books e.g. a howler which is a nasty letter sent to tell someone off; this is not the usual use of the word, howler of course
When a new word enters the lexicon this is known as lexicalization. An example would be the use of howler above or when neologisms, having been used for some time and gained widespread acceptance, are more “formally” accepted and included in dictionaries and so on.
A Lexicon in Linguistics
Linguistically speaking, a lexicon is a list of lexemes rather than words.
A lexeme is, of course, a unit of language with a distinct meaning; it can be equated to the headword in a dictionary so there is one lexeme with a number of forms:
lexeme forms: eat, ate, eaten, eating
In linguistics then, a lexicon will include the word eat but not the various forms.
In addition, we generally use the word lexicon as specific to one language only. So, for example, if you spoke English, French and Russian you would have 3 lexicons.
Lexicon and TEFL
Should you talk about lexicons with your ESL class?
Probably not. To keep things simple stick to talking about words and more generally, vocabulary.
You have an excellent vocabulary!
is more easily understood by most learners than
You have an excellent lexicon!
Lexicon comes from Medieval Latin which itself comes from Ancient Greek λεξικόν (leksikon) which comes from λέξις (leksis) which comes from λέγειν (legein) which means to speak.
Lexeme – more on lexemes, the words which go to make up a lexicon
Vocabulary and TEFL– a general look at vocabulary in English
Words in English – what it means to be a word in English
The Harry Potter Lexicon – a site devoted to the esoteric lexicon of Harry Potter