WordNet (r) 3.0 (2006):

disk operating system
    n 1: an operating system that is on a disk [syn: DOS, disk
         operating system]

The Free On-line Dictionary of Computing (30 December 2018):

disk operating system

    (DOS) The name of a number of operating
   systems which include facilities for storing files on disk,
   often used to refer to Microsoft DOS.  Such a system must
   handle physical disk I/O, the mapping of file names to disk
   addresses and protection of files from unauthorised access (in
   a multi-user system).

   A DOS should present a uniform interface to different storage
   device such as floppy disks, hard disks and magnetic
   tape drives.  It may also provide some kind of locking to
   prevent unintentional simultaneous access by two processes to
   the same file (or record).

   (1998-07-08)


The Free On-line Dictionary of Computing (30 December 2018):

Disk Operating System

    (DOS) The original disk operating system
   from IBM.

   DOS was the low-end OS of choice on the IBM 360, the
   high-end system was called just "OS".  DOS had a smaller
   kernel and less functionality than OS and could run on the
   typical 32K 360/30 and 64K 360/40 class machines.

   DOS was a successor to TOS.

   (1999-01-20)