Horizon

The horizon of a system is a conceptual group of elements which rarely interact with each other when compared to other elements of the same system, or that have interacted with the system in some meaningful capacity only in a distant past.

The horizon is not a physical location, but an abstract grouping of elements that share a common property of weak cohesion with the system.

It is crucial to understand that this does not mean these elements lack interaction with any other elements of any other system. Their belonging to the horizon of a given system is not, necessarily, an indication that they are isolated in a universal sense. They can still hold strong cohesion with other systems.