Dictionary in iBooks

Not sure what you mean, the dictionary is still there, it still get's invoked from iBooks by highlighting the word and tapping on "Define". What's your problem?

Oops! Now I see what you mean! I had only tried it with iBooks on my iPhone where it is working fine, but on the iPad every word comes back with "no definition found". Hmm, what's going on with this...

Okay easyily fixed. For some reason on the iPad it hadn't downloaded any of the available dictionaries.

To fix it select a word in a book and tap "Define", when it come up with a dialog saying "no definition found" at the bottom of that message bubble/dialog box, on the left is a button that says "Manage". Tap that and you'll get a list of all the available dictionaries -- tap on the iCloud icons for one(s) you want to download and install. I selected the Apple Dictionary, the New Oxford American Dictionary and the Oxford Dictionary of English.

but the iCloud icon is itself missing.

What language are you using? What version of iOS are you using? iPhone or iPad?

My previous instructions were for iOS 7, but it seems to work the same with iOS 8. When you tap the "Manage" button it lists all available dictionaries, the currently installed ones have an "x" in a black circle next to them on the right and all other available ones should have the iCloud icon with the downward pointing arrow instead. What dictionaries do you see listed when you tap on "Manage" button?