<filename extension> The portion of a filename, following the final point, which indicates the kind of data stored in the file. Many operating systems use filename extensions, e.g. Unix, VMS, MS-DOS. They are usually from one to three letters (some sad old OSes support no more than three). Examples are such as "c" for C source code, "ps" for Postscript, "txt" for arbitrary text.
(05 Jan 1995)