Information at the beginning of data which describes or defines the data
In information technology, header is supplemental data placed at the beginning of a block of data being stored or transmitted. In data transmission, the data following the header is sometimes called the payload or body.
It is vital that header composition follows a clear and unambiguous specification or format, to allow for parsing.
Examples
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.
In a data packet sent via the Internet, the data (payload) are preceded by header information such as the sender's and the recipient's IP addresses, the protocol governing the format of the payload and several other formats. The header's format is specified in the Internet Protocol, see IP header.