Lexicon
Etymology
From Greek lexikon (biblion), "word-book," the neuter of lexikos ("of words"), from lexis ("word, speech"), from legein ("to speak"). It entered English in the early seventeenth century meaning a dictionary, especially of Greek, Hebrew, or Latin. From "a book of words" it broadened to "the body of words itself" — the sense in which a subculture has its own lexicon.
noun · /LEK-sih-kon/
The full stock of words available in a language, a field, or a single person’s vocabulary.
What it names
The complete word-inventory behind speech — whether a language’s entire wordstock or the particular set of terms one community actually draws on.
Example
A year in the startup world added "runway," "pivot," and "moat" to her lexicon.
Related idea
A lexicon is the whole stock of words in play; diction is a speaker’s or writer’s actual choice among them. Two poets can share a lexicon yet have utterly different diction.