Software (which might be implemented as firmware) used by a computational device to manage the hardware and provide a logical interface for Application Software to request I/O services or task management services. By itself, an OS (e.g., Unix, Windows, Linux) usually has little utility to an End User until application software is installed.
An OS typically functions to isolate the Application Software from the minutiae of hardware access, so that any given piece of Application Software need not have hardware-specific aspects (see device driver).