An operating system residing in the main memory on a PC.
Disk Operating System (specifically) and disk operating system (generically), most often abbreviated as DOS (not to be confused with the DOS family of disk operating systems for the IBM PC compatible platform), refer to operating system software used in most computers that provides the abstraction and management of secondary storage devices and the information on them (e.g., file systems for organizing files of all sorts).
Such software is referred to as a disk operating system when the storage devices it manages are made of rotating platters (such as hard disks or floppy disks).
In the early days of microcomputing, memory space was often limited, so the disk operating system was an extension of the operating system.
This component was only loaded if it was needed. Otherwise, disk-access would be limited to low-level operations such as reading and writing disks at the sector-level.
In some cases, the disk operating system component (or even the operating system) was known as DOS.
Sometimes, a disk operating system can refer to the entire operating system if it is loaded off a disk and supports the abstraction and management of disk devices.
An example is DOS/360. On the PC compatible platform, an entire family of operating systems was called DOS.