Port mirroringis a technology that sends a copy of the packets seen on one switch port (or an entire VLAN) to a network monitoring connection on another switch port.
We use port mirroring to analyse network data. It can be used to mirror either inbound or outbound traffic (or both) on single or multiple interfaces.
A port mirror may also be referred to as a SPAN port - a Cisco-specific term, but often used generically.