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Dictionary.com is actually part of Reference.com (property of Ask.com) which includes a dictionary, encyclopedia, thesaurus and Ask's web search, as well as other content such as daily puzzles, grammar info, crossword solver and the like. The iPhone version includes the dictionary and the thesaurus, as well as audio pronunciations, similarly spelled words and word of the day. In numbers, it offers 275,000 definitions and 80,000 synonyms.
Dictionary.com now joins the ranks of many other iPhone dictionary applications. Some of these are very good; most notably, Merriam-Webster's collegiate dictionary, (which costs $24.99) but Dictionary.com is free and beats most of the competition if you look at sheer number of entries.