means A lexicon is the complete vocabulary of a language, a field, or a single person — every word available to be used.

from Straight from Greek 'lexikon,' a neuter form of 'lexikos' meaning 'of words,' built on 'lexis' (word, speech), which grows from the verb 'legein,' to speak. That same 'legein' is a busy ancestor — it also feeds 'logos' and a whole family of '-logy' words. So a lexicon is, quite literally, a 'thing of words,' arriving in English by way of scholarly Latin around the 17th century, when it first meant a dictionary for ancient languages.