What defines a communications system created by linking two or more devices?

A communications system that is established by linking two or more devices is referred to as a network. This is fundamental to understanding how devices interact and share information within various environments, such as homes, offices, or extensive organizational infrastructures.

Networks can be as simple as a connection between two computers or as complex as the internet, which interconnects millions of devices across the globe. The primary function of a network is to enable communication — allowing data to be transmitted between devices efficiently and effectively.

In contrast, software refers to the applications and operating systems that run on devices, while hardware relates to the physical components of computers and other devices. A control unit, on the other hand, is a part of a computer's architecture responsible for directing its operations, but it does not pertain specifically to the linking of multiple devices for communication. Thus, among the given options, "network" distinctly encapsulates the concept of linking devices for communication purposes.