Dictionary.com is one application that everyone should have on their iPad. Dictionary.com has 1,000,000 words, 90,000 synonyms, antonyms and since it caches everything right on your iPad, there's no need to be connected to the internet to use it. There is also a audio feature so you can ensure you will pronounce the words properly, and a word of the day -- with its definition explained -- to help grow your vocabulary.
The interface of Dictionary.com is very clean and easy to access. The main screen has a search bar for the thesaurus or the dictionary. Once the word is written you have a pull down list of words to choose from. The word is then displayed with the definitions. The words origin, history, phonetic and audio pronunciation are also accessible from this screen. The thesaurus is similar in format, with a simple search bar and pull down menu of words to access.