
Mediawiki Encyclopedia.
Wikipedia API is a free Open Data API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
Details from mediawiki.org.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Composite travel-safety scores for 180 countries (FCDO + US State + GPI + WGI + live incident wire), OpenAPI 3.1 spec, CC BY 4.0
Brazilian company profiles by CNPJ, CNAE establishment counts and municipality coordinates
The Wikipedia API is provided by MediaWiki (mediawiki.org).
Yes. The Wikipedia API is listed in the Open Data 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.
No. The Wikipedia API requires no authentication, so you can start making requests without signing up.
CORS support for the Wikipedia API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Wikipedia API is served over HTTPS, so it's safe to use in production.