A means of establishing an outbound-connection through a firewall in such a way that it is neither blocked nor monitored. If a cracker has compromised a machine on the other side of a firewall, a tunnel will allow the cracker to communicate with that machine from the Internet. More generally, a tunnel is a path established by 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 allows organizations to send information across a virtual private network (VPN) using the Internet as a transport medium. Tunneling is accomplished by embedding its own network protocol within the TCP/IP packets carried by the Internet.
Graham, R. Hacking Lexicon. [Online, 2001.] Robert Graham Website. http://www.linuxsecurity.com/resource_files/documentation/hacking-dict.html.