(1) In a disk or tape file, a set of data that resides permanently at the beginning. It may be used for identification only (type of file, date of last update, etc.), or it may describe the structural layout of the contents, as is common with many document and database formats.
(2) In a document or report, common text printed at the top of every page.
(3) Any caption or description used as a headline.
(4) In communications and networking, a group of fields added to the beginning of the message or packet. The fields typically include source and destination addresses as well as data that describe the message content. See
HTTP header,
email header and
header file.