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in metalworking. (1) Hot deformation, in which the length of a billet is increased by decreasing the area of its cross section. Extension is performed on hammers and presses by successive pressings of the billet, with a rotation of 90° after each pressing.
(2) Cold sheet pressing, in which hollow components are made from sheets. Cold extension is used to make bushings, casings for instruments and apparatus, and cans and saucepans. The billet is usually cut from a sheet, with an allowance for diameter, and is pressed into the peripheral area of the matrix. Upon the subsequent movement of the puncheon, the middle, unpressed part of the billet is pressed into the opening in the matrix.
extension(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 that catalog many more, including the most obscure; for example, visit www.filext.com. See Win Show file extensions, dangerous extensions and graphics formats. See also domain extension.