A Martian address is a source or destination IP address that does not exist on Planet Earth and is, thus, invalid and/or non-routable. Martian addresses are typically created accidentally by misconfigured systems, but they can also belong to a pool of reserved IANA (Internet Assigned Numbers Authority) addresses. In this context, a Martian address may also be referred to as a bogon.
Martian packets were first mentioned in 1995 in RFC 1812, Requirements for IP Version 4 Routers. Today, most networks filter for Martian addresses. Once an IP address has been found invalid, the address is rejected and Martian packets are dropped. Filtering prevents IP spoofing and ensures that attackers can't use Martian addresses as the preliminary step for a denial of service (DoS) or man-in-the-middle (MitM) attack.
It is important to remember that IANA is constantly updating and assigning new address space and today's invalid IP address may be a legitimate destination some time in the future. When this happens, the address ceases to be Martian.
Scott Morris explains why Martian packets never make it through the routing table.