Why is Microsoft Disk Operating System so called?

DOS commonly refers to the family of closely related operating systems which dominated the IBM PC compatible market between 1981 and 1995 (or until about 2000, if Windows versions 95, 98, and Me are included) : PC-DOS, MS-DOS, FreeDOS, DR-DOS, Novell-DOS, OpenDOS, PTS-DOS, ROM-DOS and several others. They are single user, single task systems. MS-DOS from Microsoft was the most widely used. These operating systems ran on IBM PC type hardware using the Intel x86 CPUs or their compatible cousins from other makers. MS-DOS is still common today and was the foundation for many of Microsoft's operating systems (from Windows 1.0 through Windows Me). MS-DOS was later abandoned as the foundation for their operating systems.