WordWeb is a handy utility that gives you the definition, synonyms, and antonyms of any given word without requiring you to go online, though it can connect
This thesaurus/dictionary can be used to look up words from almost any program. In addition to displaying sense definitions and synonyms, WordWeb can find sets of related words.
The database has more than 160,000 root words and 222,000 senses many proper nouns, pronunciations, and usage tags. WordWeb works off line, but when online you can also quickly view Web references such as the Wikipedia encyclopedia.