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 most native English speakers whilst "imprecate" is not, even though the two words are synonymous.
The richness of a person's vocabulary is popularly thought to be a reflection of either intelligence or level of education. Accordingly, many standardised tests, such as the SAT, have questions that test vocabulary.
By words," Aristophanes said 2,400 years ago, "is the mind excited and the spirit elated." The first verse of the Gospel According to John says that "In the beginning was the Word," and all my life, words have fascinated me. I collect words the same way other people collect stamps or coins. Between 1973 and 1993, I put together a book of almost three hundred pages for my own amusement of new words that I've encountered. To no small degree, my world is nothing more or less than my words. As my vocabulary expands, so too does my ability to reflect, to remember, and to choose.