A CORS preflight request is a CORS request that checks to see if the CORS protocol is understood and a server is aware using specific methods and headers.
It is an 
OPTIONS
 request, using three HTTP request headers: 
Access-Control-Request-Method
, 
Access-Control-Request-Headers
, and the 
Origin
 header.
A preflight request is automatically issued by a browser and in normal cases, front-end developers don't need to craft such requests themselves. It appears when request is qualified as "to be preflighted" and ommited for simple requests.
For example, a client might be asking a server if it would allow a 
DELETE
 request, before sending a 
DELETE
request, by using a preflight request:
OPTIONS /resource/foo 
Access-Control-Request-Method: DELETE 
Access-Control-Request-Headers: origin, x-requested-with
Origin: https://foo.bar.org
If the server allows it, then it will respond to the preflight request with an 
Access-Control-Allow-Methods
 response header, which lists DELETE:
HTTP/1.1 204 No Content
Connection: keep-alive
Access-Control-Allow-Origin: https://foo.bar.org
Access-Control-Allow-Methods: POST, GET, OPTIONS, DELETE
Access-Control-Max-Age: 86400
The preflight response can be optionally cached for the requests created in the same url using Access-Control-Max-Age header like in the above example.

See also

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Source: https://developer.mozilla.org/en-US/docs/Glossary/Preflight_request
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