Merriam-Webster Dictionary.

America's most useful and respected dictionary. In addition to all the definitions from Merriam-Webster's Collegiate Dictionary, the app includes an integrated thesaurus, example sentences, Word of the Day, and Favorites -- along with over 1000 graphical illustrations, and over 20,000 entries covering people, places, and foreign terms. A great tool for reference, education, and vocabulary building. No internet connection is required to view any of the definitions (although you do need a connection to view illustrations and hear audio pronunciations).

• Integrated Thesaurus: Synonyms & Antonyms
• Word of the Day
• Biographic and Geographic entries
• Foreign words and phrases used in English
• Graphical Illustrations
• Favorites
• Recent History - lets you keep track of the words you've looked up