Tunnel

From: Wikipedia:

An IP tunnel is an Internet Protocol (IP) network communications channel between two networks. It is used to transport another network protocol by encapsulation of its packets.

IP tunnels are often used for connecting two disjoint IP networks that don't have a native routing path to each other, via an underlying routable protocol across an intermediate transport network.

The most common type of tunnel used to connect AMPRNet subnets are IPENCAP tunnels