MS-DOS (pronounced “em ess doss”) stands for Microsoft disk operating system, the most widely-used operating system for IBM PC and compatible computers (an operating system is the master control software program that runs the computer itself). This means that MS-DOS is the most widely used computer operating system, period, since there are something like 80 to 100 million PCs in the world, and most of them use MS-DOS. There are at least two other versions of DOS that are compatible with MS-DOS (meaning they work the same way and run the same programs). So please see the definition for DOS for the whole story.
The history of MS-DOS is a fascinating study in how business success often depends more on good timing, a nose for a good deal, and aggressive marketing, than on the technical merits of your product. When IBM first developed the IBM PC, it wanted to license a crude operating system called CP/M, which was the dominant one for personal computers at the time (ever hear of a Kaypro or an Osborne?) Apparently IBM’S offer was too low for the makers of CP/M, so IBM went shopping at Microsoft. Microsoft didn’t have a suitable product at the time, but the boss, Bill Gates could smell money. He bought out a little company that was making an imitation version of CP/M, and rushed out a revision that worked on IBM’S machine. When the IBM PC caught on, Bill got really rich-even though MS-DOS is really not so hot.
Guy Kawasaki says that MS-DOS stands for Microsoft Seeks Domination of Society.