(1) See
domain extension.
(2) A software add-on. For example, extensions add functionality to Web browsers. See
browser extension.
(3) Prior to Mac OS X, an executable module that enhanced the Mac operating system. The Windows counterpart is a "dynamic link library" (see
DLL).
(4) Apple enhancements starting with iOS 8 and OS X Version 10.10 that enable apps to share functions in other apps. See "iOS 8" in
iOS versions.
(5) A file type that is appended to the end of a file name. All executable programs in the Windows and Mac worlds use extensions: .EXE in Windows; .APP in Mac (see
APP file). In the Unix/Linux environment, "executables" do not use an extension, but no matter which environment, "data" files have extensions. For example, a file with a .DOC or .DOCX extension is a Microsoft Word document. A file with a .JPG extension is a JPEG image.
Prior to Windows 95, extensions were limited to three characters. Starting with Windows 95, they can be very large (254-260 characters depending on Windows version); however, they are kept small in practice.
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 websites such as FileExt that catalog many more. See
Win Show file extensions,
dangerous extensions and
graphics formats. See also
domain extension.