WordNet® is a large lexical database of English, developed under the direction of George A. Miller. Nouns, verbs, adjectives and adverbs are grouped into sets of cognitive synonyms (synsets), each expressing a distinct concept. Synsets are interlinked by means of conceptual-semantic and lexical relations. The resulting network of meaningfully related words and concepts can be navigated with the browser. WordNet is also freely and publicly available for download. WordNet's structure makes it a useful tool for computational linguistics and natural language processing.
http://www.hyperdictionary.com/
HyperDictionary.com is a continuation of an educational project to provide free dictionary services to any user on the web. The current version of HyperDictionary.com is new and improved both in design and content. We have given the website a complete makeover to make it look trendy and attractive. We made it easier to use with results being displayed in a more user-friendly way. And, best of all, we have added new dictionaries to the website to provide our users with the content that they need.