Vocabulary
Vocabulary refers to the choice of words available in a language, or to a speaker's grasp of it.
Vocabulary often refers to words rather than concepts.
Usually a good set of words are complex synonyms for simpler ones,
or are words for very specific and not regularly required concepts, say, defenestration.
Almost all speakers have a rather limited vocabulary compared to the total language's existing words - but five to ten thousand words tend to cover (on the order of) 90% of practical use, when (apparently(verify)) going by token count of every-day usage corpora.
See also compounds, as they can hugely expand a language's vocabulary (and make its size ill defined) and complexifies automated interpretation of unseen words.
Inflection and derivation have similar effect.