an alphabetical list of words relating to a specific subject, text, or dialect.
When a client's browser sends a request to a web server, then the response given by the web server in the form of text is known as Header Response. For example, if the browser requests a URL of a website which is not available, then the server gives a header response as "404 Not Found". There are various Header Responses defined as per the requests.
200's - Successful requests
300's - Redirections
400's - Problem with the request
500's - Problem with the server