iAnnotate integrates with the native iOS Dictionary to provide the most accurate definitions. Tap and hold on a word you'd like defined then choose "Dictionary" from the popup menu that appears.
To install a dictionary, long tap on a word and select Define from the popup menu. If you see "No Definition Found" when trying to define selectable text, you will need to install a dictionary pack from Apple.
Tap "Manage" in the lower left of the dictionary popup window and locate the dictionary you'd like to install, then tap the download icon.