Abbreviation for Operating System. An operating system is the basic set of instructions that defines the behavior of something. Windows, DOS, OS2, Linux, and Mac OS are examples of operating systems that define the operation of computers. All home computers use one or more operating systems. Other programs for specific tasks may be loaded in the foreground, but the OS is always the underlying mechanism that makes everything work. Operating Systems are also part of most sophisticated electronic equipment. Synthesizers, calculators, automobiles, CD players, Palm Pilots, and any number of other household items all have underlying operating systems that define their operation.