WordNet® is an online lexical reference system whose design is inspired by current psycholinguistic theories of human lexical memory. English nouns, verbs, adjectives and adverbs are organized into synonym sets, each representing one underlying lexical concept. Different relations link the synonym sets
Very usefull on-line (and downloadable) eglish dictionary, that lists synonyms, coordinate terms, hypernyms, hyponyms, holonyms, meronyms and familiarity