DiskOperatingSystem is a general term for an OperatingSystem that provides abstractions for and controls access to the computers disks and other secondary storage devices. CP/M and many other retro OperatingSystems are DiskOperatingSystems.
MicroSoft has created several DiskOperatingSystems:
MsDos1.x
MsDos2.x
MsDos3.x
MsDos4.x
MsDos5.x
MsDos6.x
MsDos7.x (which was made for Win95 and Win98)
Also see: http://foldoc.doc.ic.ac.uk/foldoc/foldoc.cgi?disk+operating+system