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A record sent by clients and servers communicating with each other via the HTTP protocol. The header is a stream of text that may be sent without any content following it or with the content that it describes. There are headers specific to requests and to responses and others that are used for both client and server to describe or query the content or environment. An example of a common request header sent to the server is If-Modified-Since. The server returns the file only if it has been changed since a certain date and time. See HTTP.