From WordNet (r) 1.6 [wn]:

  WordNet
       n : a machine-readable lexical database organized by meanings
           [syn: {WordNet}]

WordNet provides many of the initial definitions for words in this database. Available at http://www.cogsci.princeton.edu/~wn/ it describes itself as follows: "WordNet® is an on-line 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."