Hans Wehr is the best Arabic-to-English dictionary and I believe that this is the English-to-Arabic equivalent of Hans Wehr. The only thing that is kind of confusing is that I think it was designed for Arabic speakers who are trying to learn English, because significant portions of the explanatory material are in Arabic.

But on the plus side, this dictionary is huge and comprehensive and has cool little sections on famous people (if you have ever needed to correctly spell Nietzsche or Marx in Arabic it comes in handy!), Arabic proverbs, and Arabic words that have been loaned into English. I tend to use it in conjunction with Hans Wehr – looking up the word in the English-to-Arabic and then confirming with the Arabic-to-English that I’ve got the correct word for the meaning I want to express.