A synonyms database is a "thesaurus".<p>The Moby Thesaurus II "contains 30,260 root words, with 2,520,264 synonyms and related terms", and is public domain:
<a href="https://en.wikipedia.org/wiki/Moby_Project#Thesaurus" rel="nofollow">https://en.wikipedia.org/wiki/Moby_Project#Thesaurus</a><p>The word lists can be downloaded on Project Gutenberg:
<a href="http://www.gutenberg.org/catalog/world/results?title=moby+list" rel="nofollow">http://www.gutenberg.org/catalog/world/results?title=moby+li...</a><p>There's also some public domain software associated with it: <a href="https://github.com/words/moby" rel="nofollow">https://github.com/words/moby</a>