A device file is a reserved keyword used in MS-DOS and MS-DOS–based systems to allow access to certain ports and devices.
MS-DOS uses device files for access to printers and ports. Most versions of windows also contain this support, which can cause confusion when trying to make files and folders of certain names, as they cannot have these names. A common misconception is that these are bugs which Microsoft has failed to fix.
Device file system - Wikipedia, the free encyclopedia