Short for Internet Protocol. A protocol that specifies the way data is broken into packets and the way those packets are addressed for transmission. See more at TCP/IP.
(IP) The network layer for the TCP/IP protocol suite widely used on Ethernet networks, defined in STD 5, RFC 791. IP is a connectionless, best-effort packet switching protocol. It provides packet routing, fragmentation and re-assembly through the data link layer.
IPv4 is the version in widespread use and IPv6 was just beginning to come into use in 2000 but is still not widespread by 2008.
[Other versions? Dates?]