IPad built-in dictionary
It is not available to developers. You could use an open source dictionary like this one from Project Gutenberg , but it might require a significant change to make it presentable.
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Unfortunately, I believe the system dictionary is only accessible via a private API.
Personal hypothesis: This is one of many "features" locked away by Apple to separate the iBooks app from a third-party ebook reader competition. However, at Apple's rate, I expect the API to go public at some point in the next few years.
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I think it might be a copyright problem. I had the same problem about a year ago and hasn't changed much since then.
My solution was to embed a standalone dictionary in my application, and later I turned it into a static library that others can use (for a small fee). If you are looking for a full-blown dictionary you can check it out at www.lexicontext.com
If you just need a spell checker, you can also use UITextChecker
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