Whether this has been around for some time or not, I don't know, but it's pretty certain very few people know about it. Google has a dictionary. The app provides definitions and synonyms in almost thirty languages, and each word is pulled from a scope of different sources.
Words can be 'starred' so you can come back to it for whatever reason, you can view your recent searches, and you easily switch to other languages if necessary.
Check it out: Google Dictionary