Natural Language Processing.
Dialogflow API is a free Machine Learning API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
A natural language understanding platform that enables organizations to develop conversational user interfaces.
Details from cloud.google.com.

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Used for the primary ways for filtering the stopping, stemming words from the text data
The Dialogflow API is provided by Google Cloud Documentation (cloud.google.com). A natural language understanding platform that enables organizations to develop conversational user interfaces.
Yes. The Dialogflow API is listed in the Machine Learning 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 Dialogflow API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Dialogflow API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Dialogflow API is served over HTTPS, so it's safe to use in production.