Wordnik, a nonprofit organization, is an online English dictionary and language resource that provides dictionary and thesaurus content. Some of the content is based on print dictionaries such as the Century Dictionary, the American Heritage Dictionary, WordNet, and GCIDE. Wordnik has collected a corpus of billions of words which it uses to display example sentences, allowing it to provide information on a much larger set of words than a typical dictionary. Wordnik uses as many real examples as possible when defining a word.
Wiktionary, the free open dictionary project, is one major source of words and citations used by Wordnik.