A group of characters appended to the end of a disk file name. File extensions usually consists of a full stop (dot) and one to three characters.
The three-character suffix appended to a file name in the FAT file-naming convention. The extension is optional in some file systems. The compilers use file extensions to determine the source type of the file.
The letters following the dot in a Filename that identify the type of file. For example, Web Pages commonly have the File Extension ".html."