Request header is explained as HTTP header that could be used in HTTP request. Mind you, this does not have any relevance to whatever the content of the message is.
Note: HTTP headers are the value pairs or name displayed in request and response messages of the message headers for HTTP. The value and header name are separated by single colon. HTTP headers are vital part of the process for HTTP responses and requests.
Request headers are similar to If-*, Accept-* or Accept allows to do conditional requests; others similar to User-Agent, Referrer or Cookie precise the context. This way, the server will be able to tailor the answer.
A bit of Discretion
Keep in mind that not all headers will appear in request as a request header. Let me give you an example, the content length that appears in a post request is basically called as the entity header. This refers to the size of the body of request message.
On the other hand, these entity headers are frequently deemed as request headers in such context.
Not only that, CORS do define subset of request headers as request headers, simple headers that are considered always authorized and not listed explicitly in responses in preflight requests.