Dictionary Data.
Oxford API is a free Dictionaries API that requires apiKey authentication, is served over HTTPS and does not support CORS, so it should be called from a server.
The Oxford Dictionaries API gives you access to our world-renowned dictionary data, including definitions, translations, synonyms, and audio pronunciations.
Details from developer.oxforddictionaries.com.

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Word definitions, pronunciations, synonyms, antonyms and others
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The Oxford API is provided by Oxford Dictionaries API (developer.oxforddictionaries.com). The Oxford Dictionaries API gives you access to our world-renowned dictionary data, including definitions, translations, synonyms, and audio pronunciations.
Yes. The Oxford 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 Oxford API uses apiKey authentication, so you need to register for credentials before making requests.
Not directly. The Oxford API does not allow CORS, so call it from a server, serverless function or proxy instead of front-end JavaScript.
Yes. The Oxford API is served over HTTPS, so it's safe to use in production.