Word web is a powerful free one click English thesaurus and dictionary tool which has large collection of English words in a single program.
Word Web is a complete free offline dictionary for windows which can also look in web references such as Wikipedia.
You can find definitions, synonyms and meaning of around 150 000 words stored in the database of WordWeb.
After installation WordWeb runs automatically and sits in the system consuming the very least resources and you can launch it any time by pressing the default key ctrl+alt+w which can be further customised.