A port or end of a cable. On a motherboard, the headers are the ports attached directly to the board such as USB , Video, or sound.
A part of a packet in the TCP/IP networking protocol that contains information such as destination and source addresses.
A mail header is the beginning portion of a mail message and contains information like who the message is from, its subject, and the date.
The part of the HTTP request and response that is sent before the actual content, and that contains meta-information describing the content.