Formally, in linguistics, a lexicon is a language's inventory of lexemes. Τhe word "lexicon" derives from the Greek λεξικόν (lexicon), neuter of λεξικός (lexikos) meaning "of or for words"^[1]

Linguistic theories generally regard human languages as consisting of two parts: a lexicon, essentially a catalogue of a 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.

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, are rarely given new lexemes; 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.

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^[2] 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.^[3]

The mechanisms, not mutually exclusive, are:^[4]

• Innovation,^[5] the planned creation of new roots (often on a large-scale), such as slang, branding.
• Borrowing of foreign words.
• Compounding (composition), the combination of lexemes to make a single word.
• Abbreviation of compounds.
• Acronyms, the reduction of compounds to their initial letters, such as NASA, laser (from "LASER").
• Inflection, a morphology change with a category, such as number or tense.
• Derivation, a morphological change resulting in a change of category.
• Agglutination, the compounding of morphemes into a single word.

In complex words, constituents may be dropped.^{[clarification needed]}

Besides word formation, there are also mechanisms of lexeme change:

• Lexical replacement, the replacement of a lexeme or its sense.
• Sound change, a change such as a consonant or vowel shift localised to specific words or phonotactical combinations.
• Blocking the creation of new words while existing lexical conventions are maintained.
• Obsolescence of vocabulary; much slang, for example, typically has short lifespans.

Neologisms are new lexeme candidates which, if they gain wide usage over time, become part of a language's lexicon. Neologisms are often introduced by children in a type of kid slip.^[6] Another common source is slang and activities such as advertising and branding.

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 to a language, then, if successful, it will often diffuse across geographical boundaries.

Another mechanism involves generative devices that combine morphemes according to a language's rules. For example, the suffix "-able" is usually only added to transitive verbs, as in "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. The meaning of "armed and dangerous", for example, derives from the sum of its parts, while "armed to the teeth" involves metaphor.

Compound words that are not the semantical sum of their constituents can be interpreted through analogy, common sense and context.^[2] Compound words have simple morphological structures, where no more than one element usually requires inflection for agreement. On the other hand, they are subject to the rules of syntax and can contain gaps to hold other lexemes on which they operate. For instance:

• "another nail in" something/someone → "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 cross geographical boundaries:

• "house"–"wife", becoming housewife.
• város–háza ("city"–"hall" in Hungarian) becoming városháza.
• rot–Licht ("red"–"light" in German) becoming rotlicht.

Compounding tends to produce longer lexemes which may result in lexemes of unwieldy proportion. This is compensated by mechanisms that reduce the length of words.

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 ^[7] shape the lexicon, making it simpler to acquire and often creating an illusion of great regularity in language.

• Phonological assimilation, the modification of loanwords to fit a new language's sound structure more effectively. If, however, a loanword sounds too "foreign", inflection or derivation rules may not be able to transform it.
• Analogy, where new words undergo inflection and derivation analogous to that of words with a similar sound structure.
• Emphasis, the modification of words' stress and/or accenting.
• Metaphor, a form of semantic extension.

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 an automatic teller machine or ATM in American English would be understood by both American and British speakers, despite each group using different dialects.

When linguists study a 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 (pragmatics); language acquisition; the history and evolution of words (etymology); and the relationships between words, often studied within philosophy of language.

Various models of how lexicons are organized and how words are retrieved have been proposed in psycholinguistics, neurolinguistics and computational linguistics.

• Grammaticalization
• Lexical Markup Framework

• Aitchison, Jean. Words in the Mind: An Introduction to the Mental Lexicon. Malden, MA: Blackwell, 2003.