1.One or more lines of text that appears at the top of a page, which may include information such as the chapter number, title, or page number. If it appears at the top of every page, it is called a running header.
2.Key data for a data set or file that enables user software to interpret and process data correctly. In relation to a file, the header provides such information as number of pages, date and time and the size of the file.
For the processor, the header is particularly important in relation to programs, as it contains information that specifies the programs in the RAM.