The set of words in a language or in a particular field; vocabulary or lexicon. Often used to talk about the amount of words a person knows.
日本語の語彙を増やすには毎日単語を覚えることが大切だ。
To increase your Japanese vocabulary, it's important to learn words every day.
この教材は専門分野の語彙が多い。
This textbook contains a lot of terminology for the specialized field.
彼は語彙力が豊かで表現が上手だ。
He has a rich vocabulary and expresses himself well.