Originally codified by iso, what does the "intermediate system" in is-is (intermediate system to intermediate system) stand for?

ISO is international organisation of standardisation, it is a committee, which promotes the industrial and commercial standards world wide.

The IS-IS stands for Intermediate system to intermediate system routing protocol designed for the efficient flow of the information through a computer network system. The computer network is a group of physically connected devices, mainly computers.

The IS-IS is an IGP (interior gateway protocol) exclusively designed to use within the administrative domain. It is a link state routing protocol, which operate by flooding the information through a network of router. In this system, each router creates a database of the network's topology.