Web WordNet 1.7.1 WordNet: A lexical database for the English language
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.
WordNet was developed by the Cognitive Science Laboratory at Princeton University.

Example:

wn rule

ReferenceLanguage http://www.cogsci.princeton.edu/~wn/ Shakeel Mahate Copyright (c) 2002 David Bau Distributed under the terms of the GNU Public License, Version 2 (http://www.gnu.org/copyleft/gpl.txt)