Short for "Microsoft CD-ROM Extensions." On MS-DOS systems, a TSR software module which was loaded to provide access to CD-ROM drives. Loaded after a lower-level device driver module, MSCDEX was essentially an installable filesystem driver that understood the ISO 9660 filesystem and allowed MS-DOS to access it.

MSCDEX is still present in Windows 95, but is not usually required except for compatibility with older MS-DOS programs.