A lexicon is a list of words together with additional word-specific information, i.e. a dictionary. It is a word of Greek origin (λεξικόν) meaning vocabulary. When linguists study the lexicon, they study such things as what words are, how the vocabulary in a language is structured, how people use and store words, how they learn words, the history and evolution of words, types of relationships between words as well as how words are created.

In linguistics, a lexicon has a slightly more specialized definition, as it includes the lexemes used to actualize words. Lexemes are formed according to morpho-syntactic rules and express sememes. In this sense, a lexicon organizes the mental vocabulary in a speaker's mind: First, it organizes the vocabulary of a language according to certain principles (for instance, all verbs of motion may be linked in a lexical network) and, second, it contains a generative device producing (new) simple and complex words according to certain lexical rules. For example, the suffix '-able' can be added to transitive verbs only such that we get 'read-able' but not '*cry-able'.

Furthermore an individual lexical knowledge (or lexical concept) is a term used in academia to refer to an individual's vocabulary knowledge.

See also: lexicon (program)