The lexicon (or, roughly, the base or essential vocabulary – such as "say" but not "said, tell, told")

is the vocabulary of a language or branch of knowledge (such as nautical or medical)

The lexicon of a language is its vocabulary. Lexicon is also a synonym for a dictionary or encyclopedic dictionary

An allied science to lexicology is lexicography, which also studies words, but primarily in relation with dictionaries – it is concerned with the inclusion of words in dictionaries and from that perspective with the whole lexicon

meaningful or meaningless – for example, whether a given word is part of a language's lexicon with a generally understood meaning