System

A system is a group of interacting elements, some of which produce a strong internal cohesion, the chief property of systems.

Cohesion means that some of these interactions are not simply due to proximity or past events producing loose associations, but that this locus of interaction has consistent processes and functions.

One could explain this cohesion by stating that elements follow a same set of rules when interacting towards fulfilling this function, but elements need not be intentional or even conscious of their participation in the system for it to be called one.

It is important to note that systems are their elements and the interactions between them. There is no underlying substance to them. Meaning, though there may be elements in a system that hold more influence over it, the system itself is not a subject, but just a concept, and as such should not be treated as a force of nature or something beyond control.

The elements of a system may exchange resources, which can be anything that is meaningful for the connection between any pair of elements: information, nutrients, energy, meaning. An example of the latter is a theoretical system, where entities are abstract concepts with semantic relationships -- like the graph you are reading right now.