Accessing English dictionary on iOS

I want users to be able to look up word definitions from 开发者_开发知识库inside my iPad App. Is there a way to access the English dictionary in iOS, the way iBooks does, or do I need to go to an outside API?

I ran into the same problem when I built my first iphone app. I solved it by embedding a dictionary inside my app. Since then I've turned my solution into a static library which is available at www.lexicontext.com (note that the library costs a few bucks). I'm not aware of other solutions.