"Macmillan's Open Dictionary Provides Quality Definitions For New Words"
Also I would say that the definitions would tend to be less 'frivolous'
...Its premise seems similar to that of the Urban Dictionary; readers suggest definitions and Macmillan features those definitions on the site. However, there are a couple of notable differences between Macmillan's Open Dictionary and the Urban Dictionary. One such difference is that, while The Urban Dictionary seems to be a hodgepodge of definitions, Macmillan's Open Dictionary is more of a curated resource, composed of carefully handpicked definitions. In my stroll through the site, I could not come across any definitions that seemed badly worded or otherwise lame.
Furthermore The Urban Dictionary is filtered by our LEA and although I can't test it at the moment I think that the Open Dictionary will not be filtered [must remember to check tomorrow]
Link: Open Dictionary from Macmillan Dictionary: Free English Dictionary Online with Thesaurus
Main Macmillan dictionary link: Macmillan Dictionary and Thesaurus: Free English Dictionary Online