Wikiwords, a project to create a comprehensive and open dictionary of all terms in all languages, has bee announced recently: a new project to create an open, multilingual lexicon of all terms in all languages.
Looks like it might become such a great site as wiki's encyclopedia.
It's still on Beta and the homepage is here
. (You'll have to create an account.)
It's even now, on the very first days of it existence, already a huge dictionary!