Hard to define precisely, because operating systems arose historically as people needed to solve problems associated with using computers. Much of operating system history driven by relative cost factors of hardware and people. Hardware started out fantastically expensive relative to people and the relative cost has been decreasing ever since.
Relative costs drive the goals of the operating system.
In the early days of computer use, computers were huge machines that are expensive to buy, run and maintain. Computer used in single user, interactive mode. Programmers interact with the machine at a very low level - flick console switches, dump cards into card reader, etc. The interface is basically the raw hardware. Is very complex, and is a major source of programming difficulty.