Definitions and synonyms for more than 150,000 words.
Words API is a free Dictionaries API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.

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Yes. The Words 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 Words API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Words API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Words API is served over HTTPS, so it's safe to use in production.