An internetwork (usually given as internet for short; the use of the generic term is distinguished from references to the Internet by the use of the capital letter in the latter) is a commucation system consisting of a number of physical networks (such as Ethernets), along with a collection of routers (the current term), the packet switches which connect them into an integrated system.
An internetwork is tied together with an internetworking layer, a protocol which offers direct datagram carriage across the entire internetwork, so that it is invisible to a host using that protocol where the eventual destination is, how many (and which) networks have to be traversed to get to it, etc. It just hands its packets to the nearest router, and they take care of everything else.
Each individual physical network has a protocol which specifies how packets are carried from one interface to that network to another; these protocols will vary from network type to network type. The internetworking layer serves to insulate everything connected to the internet from these variations; it is the same for all hosts and routers throughout the internet.
All internetworking designs are heavily influenced by the ground-breaking CYCLADES network.