The software “master control application” that runs the computer. It is the first program loaded when the computer is turned on, and its main component, the kernel, resides in memory at all times. The operating system sets the standards for all application programs (such as the Web server) that run in the computer. The applications communicate with the operating system for most user interface and file management operations.
Source(s):
NIST SP 800-44 Version 2
NISTIR 7621 Rev. 1 from NIST SP 800-44 Version 2

  An integrated collection of service routines for supervising the sequencing of programs by a computer. An operating system may perform the functions of input/output control, resource scheduling, and data management. It provides application programs with the fundamental commands for controlling the computer.
Source(s):
NIST SP 800-82 Rev. 2 from The Automation, Systems, and Instrumentation Dictionary

  A collection of software that manages computer hardware resources and provides common services for computer programs.
Source(s):
NIST SP 800-152 under Operating system

  The software “master control application” that runs the computer. It is the first program loaded when the computer is turned on, and its principal component, the kernel, resides in memory at all times. The operating system sets the standards for all application programs (such as the mail server) that run in the computer. The applications communicate with the operating system for most user interface and file management operations.
Source(s):
NIST SP 800-45 Version 2

  A program that runs on a computer and provides a software platform on which other programs can run.
Source(s):
NIST SP 800-86

  A computer program, implemented in either software or firmware, which acts as an intermediary between users of a computer and the computer hardware. The purpose of an operating system is to provide an environment in which a user can execute applications.
Source(s):
NISTIR 7695 from Operating System Concepts—Third Edition