According to Wikipedia:
A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure, and purpose and expressed in its functioning.
Your digestive tract is a system. The components include: mouth, stomach, and intestines. The components work together as a system to digest food and drink. The travel industry with buses, trains, planes, UBERs, Taxis, and hotels works together as a system to allow us to travel for business or pleasure. These systems rely on a group of interacting elements that act according to a set of rules to create a unified whole.
– by biomatrix simplicity underlying complexity