A vocabulary is a set of words known to a person or other entity, or that are part of a specific language.
The vocabulary of a person is defined either as the set of all words that are understood by that person or the set of all words likely to be used by that person when constructing new sentences. So "curse" is a regular part of the vocabulary of native English speakers but "imprecate" is not, even though the two words are synonyms. The richness of a person's vocabulary is popularly thought to be a reflection of their intelligence or level of education. Accordingly, many standardized tests, such as the SAT, have questions that test vocabulary.
Increasing the size of one's vocabulary, also called vocabulary building, is generally considered to be an important part of both learning a language and improving one's skills in a language in which one is already proficient. Hence schoolchildren are often taught new words, colloquially referred to as vocab words or just vocab, as a part of a particular unit or lesson. Similarly, many adults find vocabulary building to be a fun and educational activity, as evidenced in the popularity of "word-a-day" services such as mailing lists and desktop calendars.
The word "vocabulary" is also used figuratively for qualities or techniques distinctive to a particular style, especially an architectural style.
See also