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".