Dictionary.com - Dictionary & Thesaurus - For iPad 1.0
the author of this app says:
The free Dictionary.com app delivers trusted reference content from Dictionary.com and Thesaurus.com, including nearly 1,000,000 words and definitions and 90,000 synonyms and antonyms. No Internet connection is needed. The app also features audio pronunciations, similarly spelled words and Dictionary.coms popular Word of the Day that is enjoyed by millions of people.
Requires iPhone OS 3.2 or later.