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Definition: in computer communications, line or lines of text that appear at the beginning of each e-mail or Usenet message sent across the Internet. The header includes important routing and identifying information, such as the sender's name, recipient's name (either a person or a newsgroup), date, time, and machine used when the message was composed, and the path by which the message arrived at its destination. In the case of Usenet postings, it also indicates if the message is intended for more than one group. The exact material is determined by RFC (requests for comments) and discussion. In office programs, such as word-processors and spreadsheets, the header appears above the input text and can contain information that needs to appear on every page, such as the page number. The footer performs the same function below the text.
Source: Hutchinson Dictionary of Computing and the Internet. Abingdon, Oxfordshire, GBR: Helicon Publishing, 2005. p 259.