WordNet
What is WordNet?
WordNet® is a large lexical database of English. 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.
Discontinued
The most recent Windows version of WordNet is 2.1, released in March 2005, and version 3.0 for Unix/Linux/Solaris/etc. was released in December 2006.
WordNet Screenshots
WordNet Features
Comments and Reviews
Said about WordNet as an alternative
WordNet is the basis of Artha.
But where's the download link? ;-)