Use Spotlight to look up the definition of words using the built-in iOS dictionary. No data connection required. A new row at the bottom labeled "Search for definition" is added in Spotlight.
SpotDefine uses the built-in dictionary in iOS, so you do not need a data connection to use the tweak. The developer claims a future update will bring translations, and static cell that allows you to access the feature faster.
The tweak does require iOS 7 and is compatible with 64-bit devices like the iPhone 5s. It is available for free in Cydia.