I don't know if the sandboxing nature of iOS prevents this, but I'm wondering if it would be possible to add a "Define" feature to the tap-and-hold option list? I'm reading Beveridge's translation of Calvin's Institutes, and he is constantly using words I've never heard of (and many I don't even think are actually words). One of the things I love about the Kindle and iBooks versions is the ability to look up a word as I'm reading. Although, I think I had to download a dictionary to the Kindle app.
Hopefully something y'all can think about for the future.