The three-letter code at the end of a filename that generally indicates the type of format of the file. For example, a filename in MS-DOS might be Letter1 with the extension .doc. This is written with a full stop separating the two parts of the name. The three-letter extension, .doc, shows the files is a document. Similarly, .bmp means a bitmap file, .exe means an executable program file and so on. In DOS and Windows 3.1x, the length of the filename cannot exceed 11 characters. With a three-letter extension this leaves just eight characters for the filename. In Windows 95 this has been changed.
