Defines an object which encapsulates how a set of objects interact with themselves.
Example:
A good example is a Wi-Fi router at home. All electrical devices at home (smartphone, laptop, tablet, printer)
do not exchange data directly, but by the router. Laptop -> Router -> Tablet, Tablet -> Router-> Printer and so on.
Another example is control tower, airplanes do not communicate directly with themselves but through control tower.