WordWeb is a comprehensive one-click English thesaurus and dictionary for Windows. It can be used to look up words from almost any program, showing definitions, synonyms and related words. It includes pronunciations and usage examples, and has helpful spelling and sounds-like links.

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The comprehensive English thesaurus and dictionary includes:
Definitions and synonyms
Proper nouns
Related words
Pronunciations
150 000 root words
120 000 synonym sets
Fixed web reference tabs