filename extension

A filename extension is a suffix (separated from the base filename by a dot or space) to the name of a computer file applied to indicate the encoding (file format) of its contents or usage. Examples of filename extensions are .png, .jpeg, .exe, .dmg and .txt.

Some file systems limit the length of the extension (such as the FAT file system from PC DOS/MS-DOS (without Long filename support) not allowing more than three characters and IBM’s VM/CMS not allowing more than 8) while others (such as NTFS) do not. Unix filesystems accept the separator dot as a legal filename character.