means The whole stock of words a person knows or uses, or the set of words belonging to a particular language, subject, or activity.

from From the Latin vocabulum, "a name, a designation," itself rooted in vocare, "to call" (a cousin of "voice" and "vocal"). Latin vocabularium meant a list of words, and the term arrived in English through that scholarly channel in the 1500s, first as a word-list and only later as the inner hoard each speaker carries.