MS-DOS is an abbreviation for Microsoft Disk Operating System, a computer operating system used primarily on IBM PC compatible machines. Some synonyms for MSDOS include DOS, PC-DOS, and DR-DOS, which are all variations of the same basic operating system. Other similar operating systems include FreeDOS, which is a free and open-source alternative to MS-DOS, and OpenDOS, which is a descendant of DR-DOS. Other operating systems that may also fall under the same category as MSDOS include CP/M and PC-Write, both of which were popular in the early days of personal computing. Despite the many alternatives, MS-DOS remains an influential and widely recognized operating system in the history of computing.