One of the most valuable resources of the english language is the dictionary. I won't get into the history of the dictionary or what it's done for literal society. I will note that incorporating inline definitions into reading apps is not the easiest thing to do. Sure, we now have eReaders like Apple's iBooks and the Amazon Kindle that have dictionary integration built in, but what about the rest of of the text-based universe?
Well, Dictionary.com has finally launched an API so you can have similar features in your apps, directly from Dictionary.com. While dictionary APIs are certainly nothing new, it's nice to have one from the most commonly used source for definitions.
Currently, the API is in beta and lacks quite a bit as far as documentation goes. Upon signing up, you are sent to the developer forum and that's pretty light and a little buggy, right now. The API is RESTful, yet (currently) only has an XML response and is limited to 1,000 calls per day. Hopefully that changes as they take on more API requests, as this would be a huge boost to any text-centric or reading app.
While the Dictionary.com API is still in it's beta/growing stages, it's definitely worth keeping an eye on for future integration into your app. Hopefully, things will grow as more developers sign up and begin swarming the forums with ideas and questions.