port mirroring (noun)
A network switch configuration setting that forwards a copy of each incoming and outgoing packet to a third switch port. Also known as SPAN or Switched Port Analyzer, RAP or Roving Analysis Port, and TAP or Test Access Point. When network managers and security investigators want to capture packets for analysis, they need some sort of generic TAP or Test Access Point. You can buy specialized equipment for this operation but most modern switches have this capability built in.