Just found an app today that is simple and fantastic. I got tired of typing a word into Notes to look up the definition and figured someone must have done this. It's nothing more than a front-end for the built-in dictionary in iOS 5. A search box, live suggestions while typing, and a history of words you've looked up. Great idea that's been perfectly executed. Free, .1 MB download, no Ads, launches instantly. Perfect.
http://itunes.apple.com/us/app/dictionary/id473493861?mt=8
http://itunes.apple.com/us/app/dictionary/id473493861?mt=8