Definition 1: 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. Definition 2: A part of a packet in the TCP/IP networking protocol that contains information such as destination and source addresses. Definition 3: 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. Definition 4: The part of the HTTP request and response that is sent before the actual content, and that contains meta-information describing the content.