An Acronym with two different meanings

See that page. Usually written DoS.

Disk Operating System
The original OperatingSystem for IBM's PersonalComputers, named after MicroComputer? OperatingSystems which typically had a thin hardware layer abstraction at best and didn't include any multiuser or multitasking capabilities, nor memory management. The IBM DOS does have process and memory management capabilities, but they're only very rudimentary. See also MS-DOS. FreeDOS is a Free clone.