Ingress means "the act of entering.", or 'incoming'.
Ingress Filtering is the art of filtering packets as they go into a system, as opposed to filtering them on the way out. 
RFC:2267 addresses this in light of "Defeating Denial of Service Attacks which employ IP Source Address Spoofing".
The opposite of ingress is Egress.
See TrafficShaping.
2 pages link to Ingress:
