Synonyms, thesaurus and antonyms information for any given word.
Synonyms API is a free Dictionaries API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
The Synonyms API from STANDS4 enables you to programmatically access Synonyms, Thesaurus and Antonyms information for any given word.
Details from synonyms.com.

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The Synonyms API is provided by Synonyms API (synonyms.com). The Synonyms API from STANDS4 enables you to programmatically access Synonyms, Thesaurus and Antonyms information for any given word.
Yes. The Synonyms API is listed in the Dictionaries category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Synonyms API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Synonyms API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Synonyms API is served over HTTPS, so it's safe to use in production.