Formal:
* Lexicographer: This is a person who studies and writes about words. They are experts in vocabulary and language.
* Logophile: This term literally means "lover of words." It describes someone who is passionate about words and enjoys learning them.
* Erudite: This word refers to someone who has a deep and extensive knowledge of a particular subject, which could include vocabulary.
Informal:
* Wordsmith: This is a playful term for someone who is skilled at using words.
* Verbalist: This term refers to someone who is skilled in using language.
* Vocabulary enthusiast: This is a straightforward way to describe someone who enjoys expanding their vocabulary.
Ultimately, the best term depends on the context and what you want to emphasize.