header1. The portion of a packet, preceding the actual data, containing source and destination addresses, error checking and other fields. 2. The part of an electronic mail message or news article that precedes the body of a message and contains, among other things, the sender's name and e-mail address and the date and time the message was sent.
Head Disk Assembly ♦ header ♦ Head Normal Form ♦ head normalisation theorem
Try this search on Wikipedia, Wiktionary, Google, OneLook.