In typography, any text that appears at the top of a page but is not part of the body text, such as a title, author, chapter title, etc. A header appearing on every page is called a running head.
In computing, header is a shorthand term for header information. See Header Information.
In digital imaging, a header is also a term for information located at the beginning of a sector on a CD-ROM, used to quickly access data for playback.