Definition:
(1) The collection of words--the internalized dictionary--that every speaker of the language has.
(2) A stock of terms used in a particular profession, subject, or style.
Etymology:
From the Greek, "word, speech"
Examples and Observations:
• The lexicon of soccer ("football" outside the U.S.) includes terms such as linesman, friendly match, yellow card, penalty shootout, pitch, result, and draw.

• Words by the Numbers
- "[T]here are currently about 600,000 words in the English language, with educated adults using about 2,000 words in daily conversation. For the 500 most frequently used words, there are some 14,000 dictionary meanings."
(Wallace V. Schmidt et al., Communicating Globally. Sage, 2007)

- "The English lexicon grew by 70 percent from 1950 to 2000, with roughly 8,500 new words entering the language each year. Dictionaries don't reflect a lot of those words."
(Marc Parry, "Scholars Elicit a 'Cultural Genome' From 5.2 Million Google-Digitized Books." The Chronicle of Higher Education, December 16, 2010)

• Myths of Word Learning
"If you attend a class on language acquisition, or read any good introductory chapter on the subject, you are likely to learn the following facts about word learning. Children's first words are odd; they have funny meanings that violate certain semantic principles that hold for adult language and are learned in a slow and haphazard way. Then, at about 16 months, or after learning about fifty words, there is a sudden acceleration in the rate of word learning--a word spurt or vocabulary explosion. From this point on, children learn words at the rate of five, ten, or even fifteen new words a day.

"I will suggest here that none of these claims are true. They are myths of word learning. There is no reason to believe that children's first words are learned and understood in an immature fashion--and there