Disk Operating System. A widely used interpreter or program that translates user commands into machine code for computer-disk interactions. Functions include the organization of files into folders or directories and the finding, erasing, coping, or restoring of files.

The first Microsoft Windows operating system was really an application that ran on top of the MS-DOS operating system. Today, Windows operating systems continue to support DOS (or a DOS-like user interface) for special purposes by emulating the operating system.
In the 1970s before the personal computer was invented, IBM had a different and unrelated DOS (Disk Operating System) that ran on smaller business computers. It was replaced by IBM's VSE operating system.