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Longman Dictionary of Contemporary English
operating system
noun
EXAMPLES FROM CORPUS
▪ Later in the year, a multiprocessing operating system derived from Unisys' current product will appear.
▪ None of the top 10 clonemakers now sells computers that run the Macintosh operating system.
▪ The new mainframe operating system I have personally approved is going haywire; the computer is down half the time.
▪ The toolset will include a layer that implements Windows APIs on the System 7 operating system.
▪ We like fast computers, the latest operating system, neat software and expensive peripherals as much as anyone.
Wiktionary
operating system

n. (context software English) Software that controls the allocation and usage of hardware resources such as memory, CPU time, disk space, and input and output devices.

WordNet
operating system

n. (computer science) software that controls the execution of computer programs and may provide various services [syn: OS]

Wikipedia
Operating system

An operating system (OS) is system software that manages computer hardware and software resources and provides common services for computer programs. The operating system is a component of the system software in a computer system. Application programs usually require an operating system to function.

Time-sharing operating systems schedule tasks for efficient use of the system and may also include accounting software for cost allocation of processor time, mass storage, printing, and other resources.

For hardware functions such as input and output and memory allocation, the operating system acts as an intermediary between programs and the computer hardware, although the application code is usually executed directly by the hardware and frequently makes system calls to an OS function or is interrupted by it. Operating systems are found on many devices that contain a computer from cellular phones and video game consoles to web servers and supercomputers.

Examples of popular desktop operating systems include Apple OS X, Linux and its variants, and Microsoft Windows. So-called mobile operating systems include Android and iOS. Other classes of operating systems, such as real-time (RTOS), also exist.