1. In information technology, header refers to supplemental data placed at the beginning of a block of data being stored or transmitted. In data transmission, the data following the header are sometimes called the payload or body. It is vital that header composition follows a clear and unambiguous specification or format, to allow for parsing.
2. In a computer file, a header may be a field that precedes the main file content and describes the length of the content or other characteristics of the file.
https://en.wikipedia.org/wiki/Header_(computing) (1.); http://whatis.techtarget.com/definition/header (2.)
Author: Tim Röder