System

A system is defined to be a collection of things that are related to each other. Therefore, all systems can be defined as a tuple of set of things, and a set of relations.

A system $S$ can be defined as a tuple: $\{T,R\}$, 

where

$T$ is a collection of things, $T=\{t|t\in T\}$ and 
$R$ is a set of relations that relates two things $t\in T$.