(1) In many disciplines of computer science, a header is a unit of information that precedes a data object. In a network transmission, a header is part of the data packet and contains transparent information about the file or the transmission. In file management, a header is a region at the beginning of each file where bookkeeping information is kept. The file header may contain the date the file was created, the date it was last updated, and the file's size. The header can be accessed only by the operating system or by specialized programs.
In e-mail, the first part of an e-mail message containing controlling and meta-data such as the Subject, origin and destination e-mail addresses, the path an e-mail takes, or its priority. The header will contain information about the e-mail client, and as the e-mail travels to its destination information about the path it took will be appended to the header.
(2) In word processing, one or more lines of text that appears at the top of each page of a document. Once you specify the text that should appear in the header, the word processor automatically inserts it.
Most word processors allow you to use special symbols in the header that represent changing values. For example, you can enter a symbol for the page number, and the word processor will automatically replace the symbol with the correct number on each page. If you enter the date symbol, the word processor will insert the current date, which will change if necessary each time you print the document.
Most word processors allow you to specify different headers, for example, one for odd-numbered pages (odd headers) and another for even-numbered pages (even headers). Headers are also called running heads.