Often abbreviated by the letters OS. An operating system is the basic set of instructions that defines the behavior of hardware or software. Windows, DOS, OS2, Linux, and Mac OS X 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. That includes synthesizers, calculators, automobiles, CD players, Palm Pilots, and any number of other household items, all with underlying operating systems that define their operation.