Egress filtering

Egress filtering is filtering packets which leave a network, as opposed to ingress filtering, which affects packets coming in.

Egress filtering is less useful overall, because the filtering is applied by the people who are creating the packets, and any malware which is creating those packets has the potential to reconfigure such filters to let them out.

The difference between the two has to do with administrative span of control -- you want the filters under control unrelated to the possible sources of bad packets.