WordNet (r) 3.0 (2006):

operating system
    n 1: (computer science) software that controls the execution of
         computer programs and may provide various services [syn:
         operating system, OS]

The Jargon File (version 4.4.7, 29 Dec 2003):

operating system
 n.

    [techspeak] (Often abbreviated ?OS?) The foundation software of a machine;
    that which schedules tasks, allocates storage, and presents a default
    interface to the user between applications. The facilities an operating
    system provides and its general design philosophy exert an extremely strong
    influence on programming style and on the technical cultures that grow up
    around its host machines. Hacker folklore has been shaped primarily by the
    Unix, ITS, TOPS-10, TOPS-20/TWENEX, WAITS, CP/M, MS-DOS,
    and Multics operating systems (most importantly by ITS and Unix). See
    also timesharing.


The Free On-line Dictionary of Computing (30 December 2018):

operating system

    (OS) The low-level software which handles
   the interface to peripheral hardware, schedules tasks,
   allocates storage, and presents a default interface to the
   user when no application program is running.

   The OS may be split into a kernel which is always present
   and various system programs which use facilities provided by
   the kernel to perform higher-level house-keeping tasks,
   often acting as servers in a client-server relationship.

   Some would include a graphical user interface and window
   system as part of the OS, others would not.  The operating
   system loader, BIOS, or other firmware required at boot
   time or when installing the operating system would generally
   not be considered part of the operating system, though this
   distinction is unclear in the case of a rommable operating
   system such as RISC OS.

   The facilities an operating system provides and its general
   design philosophy exert an extremely strong influence on
   programming style and on the technical cultures that grow up
   around the machines on which it runs.

   Example operating systems include 386BSD, AIX, AOS,
   Amoeba, Angel, Artemis microkernel, BeOS, Brazil,
   COS, CP/M, CTSS, Chorus, DACNOS, DOSEXEC 2,
   GCOS, GEORGE 3, GEOS, ITS, KAOS, Linux, LynxOS,
   MPV, MS-DOS, MVS, Mach, Macintosh operating system,
   Microsoft Windows, MINIX, Multics, Multipop-68,
   Novell NetWare, OS-9, OS/2, Pick, Plan 9, QNX,
   RISC OS, STING, System V, System/360, TOPS-10,
   TOPS-20, TRUSIX, TWENEX, TYMCOM-X, Thoth, Unix,
   VM/CMS, VMS, VRTX, VSTa, VxWorks, WAITS.

   FAQ
   (ftp://src.doc.ic.ac.uk/usenet/news-info/comp.os.research).

   Usenet newsgroup: news:comp.os.research.

   [Jargon File]

   (1999-06-09)