MS-DOS, the Microsoft Disk Operating System, its OEM-branded versions, and x86 DOS clones in general.
Questions regarding all versions of MS-DOS and its clones: from the first release by Microsoft for IBM (as PC DOS) in 1981 to the versions underlying Windows 9x.
Questions specific to a clone (e.g. the DR DOS family) or operating environment (e.g. the DOS box in OS/2) must indicate this in the question.