(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 a communications message, a temporary set of data that is added to the beginning of the text in order to transfer it over the network. The header contains source and destination addresses as well as data that describe the content of the message. See HTTP header, e-mail header and header file.