Formal:
* Eloquacious: This word emphasizes the person's ability to use language fluently and persuasively.
* Articulate: This describes someone who can express themselves clearly and effectively.
* Well-spoken: This is a general term for someone who speaks grammatically and with fluency.
* Learned: This implies a broad knowledge of many subjects, including language.
* Erudite: This signifies a deep understanding of a particular subject, and often suggests a love of language.
Informal:
* Wordsmith: This term refers to someone who is skilled at using words, particularly in writing.
* Lexicographer: This is a person who compiles dictionaries, which requires a vast knowledge of words.
* Logophile: This literally means "lover of words," suggesting a passion for language.
* Verbal virtuoso: This emphasizes the person's skill in using language creatively and effectively.
Other options:
* Fluent: This describes someone who speaks easily and effortlessly.
* Sophisticated: This implies a refined understanding of language and its nuances.
* Literary: This describes someone who is well-versed in literature and its language.
* Eloquent: This suggests a person who speaks beautifully and persuasively.
The best word to use depends on the context and the specific aspect of the person's vocabulary you want to highlight.