DOS is short for Disk Operating System. The term was used first by IBM some time in the sixties, but what most people think of when they hear DOS, and what we are talking about here, is the native operating system of the IBM PC, MS-DOS and compatibles.
MS-DOS is now officially dead, but DOS certainly isn't. I am not sure if people are widely aware that DOS itself never was a trademark for any company, there always were other DOSes besides Microsoft's, and anyone who feels like it can write one.
