A generic term referring to any computer software which manages and controls the hardware so that application software can perform a task.
It is an essential part of the computer system. An operating system is an obvious example, while an OpenGL or database library are less obvious examples.
System software contrasts with application software, which are programs that help the end-user to perform specific, productive tasks, such as word processing or image manipulation.
If system software is stored on non-volatile storage such as integrated circuits, it is usually termed firmware.