DOS
DOS: Denial of Service attack
DOS: Disk Operating System
'Disk Operating System' applies to any of a number of systems, the most widely-known of which is MS-DOS, the independently marketed version of PC-DOS (which was distributed with IBM PC). It was derived from QDOS (the Quick and Dirty Operating System) which, in turn, was based on CP/M. PC-DOS, as developed by Microsoft and released by IBM in 1981, was a very simple 16-[[bit] operating system without the concept of directories or any of a number of other features commonly considered essential to an operating system. Beginning with PC-DOS 2.0, some of these features were added. With DOS 6.22, the 16-bit line (with Windows 3x and under being a separate application) came to an end and DOS 7, a 32-bit extension with Windows 4x more tightly integrated, was introduced as Windows 95. That line came to an end with Windows ME, being superseded by Windows NT, which had been developed in parallel since 1993. NT is not DOS, and merely emulates it in order to continue to run some DOS applications.
An open source clone, FreeDOS exists, bearing a similar relationship to MS-DOS as Linux does to UNIX. FreeDOS is also what drives DOS applications under DOSEMU on Linux systems, by default.