A technology that enables one network to send its data via another network's connections. Tunneling works by encapsulating a network protocol within packets carried by the second network. For example, Microsoft's PPTP technology enables organizations to use the Internet to transmit data across a virtual private network (VPN). It does this by embedding its own network protocol within the TCP/IP packets carried by the Internet.
www.angelfire.com/anime3/internet/network.htm
A method by which packets are encapsulated within a protocol and a header is attached that is understood by the transporting network's routers and switches. "Tunneling" allows files to travel through the Internet as if they were traveling through a private secured network.
www2.hawaii.edu/~dflynn/VPNs/vpnglossary.htm