System software
Computer system software is any software which manages and controls pc hardware for application software to perform a task. Operating systems like Microsoft Windows and Linux are good examples of system software.
It performs tasks like transferring data from memory to disk, or rendering text onto a display device. This includes loading programs, device drivers, programming tools, compilers, assemblers and utility software.
Software libraries that perform generic functions are also viewed as system software. Almost all computers, including hand held computers, desktops, laptops, notebooks and game consoles, use an operating system.
The purpose of system software is to insulate the applications programmer as much as possible from the details of the particular computer complex being used, especially memory, other hardware features, and accessory devices such as communications, printers, readers, displays, keyboards, etc.
For a definition in the Wikipedia: pc system software.