Wordnik was founded in 2008 by Erin McKean, former editor in chief of American dictionaries at Oxford University Press. Wordnik enhances the traditional dictionary experience by allowing anyone interested in the meaning of words to participate in their definition. To supplement Wordnik's catalogue of words pulled from six different dictionaries and thesauri, including The American Heritage Dictionary of the English Language and The Century Dictionary, language enthusiasts can make suggestions for the inclusion of any word among the thousands that are omitted from print resources each year due to space limitations—and even submit invented words for consideration.
In addition to definitions, Wordnik offers users a wealth of unique data, including information about how a word is being tweeted, examples of related words that often appear in its context, and its value in Scrabble points. Like Thinkmap's Visual Thesaurus, another interactive online resource, which creates word maps, Wordnik uses visual elements to offer users innovative ways to conceive of language. Many definitions feature images from Flickr, Yahoo's photo-sharing site, that show how words are used in tags and captions, and also display charts that show how often the word has been used over time.
Here is a delightfully entertaining presentation at TED by Erin in which she redefines the dictionary. I love her statement that "Paper is the enemy of words."