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DOS /dɒs/, short for disk operating system, is an acronym used as casual shorthand for the family of several very similar command-line computer operating systems for x86-based computers: primarily MS-DOS (which first appeared in 1981 and was rebranded as IBM PC DOS until 1993), but also operating systems designed to be compatible with MS-DOS: DR-DOS (1988), ROM-DOS (1989), PTS-DOS (1993), and FreeDOS (1998). None of these systems were officially named "DOS", and indeed "DOS" is a general term for disk operating system.

MS-DOS dominated the IBM PC compatible market between 1981 and 1995, and Windows 9x (95, 98, and ME), still ran on top of it.

Dozens of other operating systems also use the acronym "DOS", including DOS/360 from 1966. Others are Apple DOS, Apple ProDOS, Atari DOS, Commodore DOS, TRSDOS, and AmigaDOS. See List of DOS operating systems § Other operating systems.

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