In most theories of linguistics, human languages are thought to consist of two parts: a lexicon, essentially a catalogue of a given language's words (its wordstock), and a grammar, a system of rules which allow for the combination of those words into meaningful sentences. The lexicon is also thought to include bound morphemes, which cannot stand alone as words (such as most affixes). In some analyses, compound words and certain classes of idiomatic expressions and other collocations are also considered to be part of the lexicon. Dictionaries represent attempts at listing, in alphabetical order, the lexicon of a given language; usually, however, bound morphemes are not included.
More formally, a lexicon is a language's inventory of lexemes. Coined in English 1603, the word "lexicon" derives from the Greek λεξικόν (lexicon), neuter of λεξικός (lexikos), "of or for words", from λέξις (lexis), "speech", "word", and that from λέγω (lego), "to say", "to speak".
Size and organization 
Items in the lexicon are called lexemes or word forms. Lexemes are not atomic elements but contain both phonological and morphological components. When describing the lexicon a reductionist approach is used, trying to remain general while using a minimal description. To describe the size of a lexicon, lexemes are grouped into lemmas. A lemma is a group of lexemes generated by inflectional morphology. Lemmas are represented in dictionaries by headwords which list the citation forms and any irregular forms, since these must be learned to use the words correctly. Lexemes derived from a word by derivation morphology are considered new lemmas. The lexicon is also organized according to open and closed categories. Closed categories, such as determiners or pronouns, rarely get new lexemes and their function is primarily syntactic. Open categories such as nouns and verbs have highly active generation mechanisms and their lexemes are more semantic in nature.
Lexicalization and other Mechanisms in the Lexicon 
A central role of the Lexicon is the documenting of established lexical norms and conventions. Lexicalization is the process where new words, having gained into widespread usage, enter in the lexicon. Since lexicalization may modify lexeme phonologically and morphologically it is possible, that a single etymological source may be borrowed in two or more forms into a single lexicon. These pairs are called doublet are often close semantically. Two examples are aptitude versus attitude, and employ versus imply.
The mechanisms (not mutually exclusive) are -
- Innovation - the planned creation of new roots (often on a large-scale), e.g. slang, branding, .
- Borrowing - of foreign words.
- Compounding also called composition is the combination of lexemes into a single word.
- Abbreviation of compounds.
- Acronyms reduction of compound to their initial letters, e.g. NASA, laser.
- Inflection morphology change with a category - such as number or tense.
- Derivation morphological change resulting in a change of category.
- Agglutination a compounding of morphemes into a single word.
- In complex words constituents may be dropped.
Besides word-formation there are also mechanism of change in an exiting lexeme.
- Lexical Replacement - replacement, either complete or in a word sense.
- Sound Change localised to specific words, phonotactics combination or systemic taking the form of a consonant or a vowel shift.
- Blocking - existing lexical conventions block creation of new words.
- Obsolescence of vocabulary - slang typically have short lifespans.
New Words 
Neologism are new lexeme candidates which if they gain wide usage overtime become part of a language's lexicon. Neologism are often introduced by children in a type of kid slip. Neologism are also introduced by adults in marketing activities such as advertising and branding, and in slang.
Loan Words 
Most innovations to a lexicon are either loan words introduced by bilingual speakers during language contact or compound words created from existing morphemes. Once a neologism or a compound is introduced in one languages if successful it will often diffuse across geographical boundaries.
The Role of Morphology 
Another mechanism involves generative devices which combine morphemes according to a language's rules. For example, the suffix "-able" can be added to transitive verbs only, so that we get "read-able" but not "cry-able".
A compound word is a lexeme composed of several pre-existing morphemes. Because a compound word is composed of established lexeme they are usually easier to acquire than loan words or neologisms. Their meaning is usually just a sum of their constituent parts.
Compound that are not the semantical sum of their of its constituents can be interpreted through analogy, common sense and context Compound words have simple morphological structures, one or no elements requiring inflection for agreement. On the other hand they are subject to the rules of syntax and can contain gaps to hold other lexems on which they operate.
- another nail in someone/something → another nail in the company's coffin
- bring something to someone's attention → bring a problem to your Attention
Once new compounds are successfully established in one language they will often diffuse across geographical boundaries. Examples
Compounding tends to produces longer lexemes which may result in lexeme of unwieldy proportion. To compensate for this there are also mechanism which reduce the length of a words
Diachronic Mechanisms 
Comparative historical linguistics studies the evolutions languages and takes a diachronic view of the lexicon. The evolution of lexicons in different languages occurs through parallel mechanisms. Over time historical forces work to  shape the lexicon, making it simpler to acquire and often creating an illusion of great regularity in language.
- Phonological Assimilation - modification of loan words to better fit a new language's sound structure. If a loan word is too foreign sounding inflection or derivation rules may not be able to transform it.
- Analogy - new words undergo inflection and derivation analogous to that of words with a similar sound structure.
- Emphasis - words are modified for effect of emphasis.
- Metaphor - a form of semantic extension.
Second Language Lexicon 
The term lexicon is generally used in the context of single language. Therefore, multi-lingual speakers are generally thought to have multiple lexicons. Speakers of language variants (Brazilian Portuguese and European Portuguese, for example) may be considered to possess a single lexicon. Thus a cash dispenser (British English) as well as and automatic teller machine or ATM in (American English) while belonging to speakers of different dialect would be understood by both American and British dialect speakers.
When linguists study the lexicon, they consider such things as what constitutes a word; the word-concept relationship; lexical access and lexical access failure; how a word's phonology, syntax, and meaning intersect; the morphology-word relationship; vocabulary structure within a given language; language use (that is, pragmatics); language acquisition; the history and evolution of words (i.e. etymology); and the relationships between words, often studied within philosophy of language.
See also 
- λεξικός, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library
- λέξις, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library
- λέγω, Henry George Liddell, Robert Scott, An Intermediate Greek-English Lexicon, on Perseus Digital Library
- Geert, Booij (2005). The grammar of words : an introduction to linguistic morphology. Oxford textbooks in linguistics. Oxford University Press. ISBN [[Special:BookSources/0-19-928042-3|0-19-928042-3 [[Category:Articles with invalid ISBNs]]]] Check
- Skeat, Walter (17-04-2010). A Concise Etymological Dictionary of the English Language. Forgotten Books. p. 648. ISBN 978-1-4400-5722-9.
- Ornan, Uzzi (2003). The Final Word — Mechanism For Hebrew Word Generation (in Hebrew). Haifa: Haifa University Press. ISBN [[Special:BookSources/965-311-054-4|965-311-054-4 [[Category:Articles with invalid ISBNs]]]] Check
- Metcalf, Allan (2002). Predicting New Words — The Secrets of Their Success. Boston: Houghton Mifflin Company. ISBN 0-618-13006-3.
- Jaeger, Jeri J. (2005). Kid's slips: what young children's slips of the tongue reveal about language development. Psychology Press. ISBN 978-0-8058-3579-3. Retrieved 8 April 2012.
- Deutscher, Guy (May 19, 2005). The Unfolding of Language: An Evolutionary Tour of Mankind's Greatest Invention. Metropolitan Books.
Further reading 
|Look up lexicon in Wiktionary, the free dictionary.|
- Aitchison, Jean. Words in the Mind: An Introduction to the Mental Lexicon. Malden, MA: Blackwell, 2003.