Packets that turn up unexpectedly on the wrong network because of bogus routing entries. Also a packet which has an altogether bogus (non-registered or ill-formed) internet address, such as the test loopback interface [127.0.0.1]. Such a packet will come back labelled with a source address that is clearly not of this earth. "The domain server is getting lots of packets from Mars. Does that gateway have a martian filter?"

[Jargon File]