1. 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.

Nearby terms:

Head Disk AssemblyheaderHead Normal Formhead normalisation theorem

Try this search on Wikipedia, Wiktionary, Google, OneLook.