This English thesaurus will show synonyms and antonyms for any word.
This app provides a free English thesaurus with both synonyms and antonyms.
Synonyms: Words that have the same, or very similar meaning.
Antonyms: Words that have opposite, or nearly opposite meaning.
For extra features (including easy right-click word lookup), install the Thesaurus Extension, too. You can find it here: http://goo.gl/6Bcxg
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