I just got off the phone with Apple support because the dictionary was not available in iBooks. I have a new iPad Mini and I used the restore from my iPad 2. I tried just about everything with no success. The Apple person had me restore my iPad Mini as a new iPad with no restoring from any backups. I then setup the Mini as a new iPad and downloaded iBooks.
Defnitions were now back! I know this isn't a great soltuion since you have to manually login to all your apps and set them up again, but at least I have definitions back and I read a lot, so this was good.
I hope this helps someone who may have used a previous backup for your new Mini or iPad 4. Not sure if this applies to the newest iPad but it did with my Mini.