WordNet® was developed at Princeton University. It is a large lexical database of English. Nouns, verbs, adjectives and adverbs, 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.
You can read more about Wordnet here. The wording is pretty technical, but you can probably best experience Wordnet in action to see it's power and functionality. A textual implementation is available here, but you can really experience its power with the visual implementation.
NOTE: While Visual Wordnet functions on a phone or small screen, it is best experienced on a desktop or tablet.
Wordnet Plus for Golden Dict
We have implemented a Golden Dict version of Wordnet called Wordnet Plus. With this edition, not only the headwords of the original are one-click enabled, but so is EVERY WORD in the sample sentences and phrases. You can also install the optional visual implementation.
CITE: Princeton University "About WordNet." WordNet. Princeton University. 2010. <http://wordnet.princeton.edu>