A lexicon is the vocabulary of a language or branch of knowledge.
A list of all the words used in a particular language or subject, or a dictionary.
Linguistic theories generally regard human languages as consisting of two parts: a lexicon, essentially a catalogue of a language’s words; 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. In some analyses, compound words and certain classes of idiomatic expressions, collocations and other phrases 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.
Wikipedia, “Lexicon”. https://en.wikipedia.org/wiki/Lexicon