A Header refers to the additional data at the beginning of a chunk of data (or packet) being stored or transmitted. The data that follows the header is called the payload or body. Note that it is important that the header is of clear and unambiguous format to allow for parsing.

E-mail header: The text (body) is preceded by header lines indicating sender, recipient, subject, sending time stamp, receiving time stamps of all intermediate and the final mail transfer agents, and much more.

See also: Header