Ingress = traffic entering the switch port; Egress = traffic leaving the switch port.

These are fundamental switching terms you'll see throughout Cisco documentation:

• Ingress — the interface where a frame arrives
• Egress — the interface where a frame is sent out

A switch never forwards a frame back out the port it arrived on. This is because the source device is already on that segment — sending the frame back would be pointless and could cause loops or duplicate traffic.

The switch uses its MAC (Media Access Control) address table (mapping destination MAC addresses to ports) to determine which egress port to use. If it doesn't know the destination, it floods the frame out all ports except the ingress port.

Go deeper:

• Host to Host through a Switch (PracticalNetworking) — the filtering behaviour that explains why a switch never sends a frame back out its ingress port.