Source : WordNet®

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

Source : Free On-Line Dictionary of Computing

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)