A SPAN port or Port Mirror is:

A technique used to send Flow data from a switch or router.

A technique used to copy packet data traversing a switch or router to a packet capture device from a passively monitoring port on the switch or router.

A technique used to connect into a live link in a network using a network-tap in order to send all packet data traversing the live link to a packet capture device.

A technique for placing a link on a switch or router in loopback mode for troubleshooting purposes.