A "Response Code" is a numerical code returned by a server in response to a client's request, typically in the context of web communication. These codes are part of the HTTP protocol and indicate the status of the request. For example, a code starting with "2" signifies success, while codes beginning with "4" or "5" indicate client or server errors, respectively. Response codes help developers and users understand what happened during a request. Common examples include 200 for a successful request, 404 for a page not found, and 500 for a server error. These codes are essential for troubleshooting and improving web applications.