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Definitions of operating system on the Web:

• (computer science) software that controls the execution of computer programs and may provide various services
  wordnetweb.princeton.edu/perl/webwn

• The THE multiprogramming system was a computer operating system designed by a team led by Edsger W. Dijkstra, described in monographs in 1965-66 ...
  en.wikipedia.org/wiki/THE_operating_system

• An operating system (OS) is an interface between hardware and user which is responsible for the management and coordination of activities and the sharing of the resources of the computer that acts as a host for computing applications run on the machine. ...
  en.wikipedia.org/wiki/Operating_system

• The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. ...
  en.wikipedia.org/wiki/(operating_system)_Linux

• Software that controls the allocation and usage of hardware resources such as memory, CPU time, disk space, and input and output devices
  en.wiktionary.org/wiki/operating_system

• A series of computer programs which control the operation of the computer itself. Application programs such as GIS software run under an operating system. Examples of operating systems include MS Windows 95, MS Windows NT, UNIX, VMS, DOS and OS/2.
  www.digimap.gg/glossary

• Program (a set of multiple programs) which enables basic communication within the computer. Without an OS, the computer is useless. ...
  www.adminxp.com/begin/index.php

• software that controls and manages hardware and basic system operations for a computer. The operating system loads programs into the computer's memory, runs these programs, and manages peripherals like disks and printers.
  www.iolo.com/labs/Glossary.aspx

• The operating system facilitates communication between the hardware (the PDA) and the software.
  www.npcentral.net/pda/pdaterms.shtml

• Software controlling the overall operation of a multipurpose computer system, including such tasks as memory allocation, input and output distribution, interrupt processing, and job scheduling.
  www.satech.com/glosofmemter.html

• System that consists of several programs that help the computer manage its own resources, such as manipulating files, running programs and controlling the keyboard and screen.
  www.tyner.com/glossary.htm

• The software that manages the hardware and software interface of a computer.
  www3.sympatico.ca/bkeevil/tapuser/gloss.html

• The most important program run on a computer because it manages all the other programs. Operating systems perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and directories on the disk, and controlling peripheral devices ...
  www.iscfr21.it/21cfr_glossary.html

• The foundation software of a machine; that which schedules tasks, allocates storage, and presents a default interface to the user between applications.
  www.portlandonline.com/Auditor/Index.cfm

• Microsoft Windows 98 Second Edition, Microsoft Windows XP Home Edition, Microsoft Windows XP Professional, Microsoft Windows NT, Microsoft Windows 2000, Microsoft Windows Server 2003
  www.fixya.com/recommendations/p13157-human_concepts_orgplus_5_standard_op5f5