lexicon
/ˈleksɪkən/·렉시콘·noun
the vocabulary of a language, person, or branch of knowledge
GreekCEFR C1
Root
Greek 'lexis' (word, speech)
Greek lexis (word) + -ikon (book of) → lexikon biblion (book of words) → Latin lexicon → English lexicon (16th c.)
In a word
Greek lexis = 'word, speech'. A scholar's root. The lex family unfolds around the act of handling words — lexicon (a book of words), lexicography (writing words = compiling dictionaries), dyslexia (words gone wrong = reading disorder). What you open when you open a dictionary is not a list — it is the Greek act of gathering words, still being performed.
Examples
Slang slowly enters the standard lexicon.
Each profession has its own lexicon.
The word "selfie" is now in the lexicon.
Related
lexicallexicographylexicographerdyslexiaalexia