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(1) See domain extension.

(2) A program module that adds functionality to an application or system program. For example, added capabilities to the Firefox browser are called "extensions" (see Firefox extension); however, the same type of add-on module for Photoshop is called a "plug-in" (see Photoshop plug-in).

     Executable modules that enhance the Mac operating system are called "extensions," while enhancements to the Windows OS are called "dynamic link libraries" (see DLL).

(3) A file type that is appended to the end of a file name. All programs and almost all data files use extensions, which are separated from the file name with a dot. For example, LETTER.DOC is a Word document. NOTEPAD.EXE is a text editor program in Windows, while TEXTEDIT.APP is a similar program in the Mac.

     In Windows versions, prior to Windows 95, an extension could have only up to three letters or digits. Starting with Windows 95, extensions could be longer, but in practice, extensions are generally limited to no more than four or five characters.

Common Extensions and Exhaustive Lists
In this encyclopedia, more than 500 common file extensions are listed under the terms "extension," followed by their first letter such as extension a, extension b and extension c. However, there are Web sites that catalog all file extensions, including the most obscure, such as FILExt at www.filext.com. See Win Show file extensions, dangerous extensions and graphics formats. See also domain extension.