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extension

(1) See domain extension.

(2) A program module that adds functionality to an application or system program. For example, program modules that add capabilities to the Firefox browser are actually called "extensions" (see Firefox extension). The same type of add-on module in the Photoshop image editor 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).

(2) 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 text editor in the Mac.

In Windows versions, prior to Windows 95, an extension could only have a maximum of three letters or digits. Starting with Windows 95, Windows files can have extensions with more characters.

Exhaustive Lists
There are Web sites that catalog as many file extensions as possible such as www.filext.com.

Popular Extensions
A list of popular extensions follows by letter. For example, to find out what an .SWF file is, look up SWF under "extension S. See Win Show file extensions, dangerous extensions and graphics formats. See also domain extension.

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extension

1. Commerce a delay, esp one agreed by all parties, in the date originally set for payment of a debt or completion of a contract

2. the property of matter by which it occupies space; size

3. Med a steady pull applied to a fractured or dislocated arm or leg to restore it to its normal position

4. 

a. a service by which some of the facilities of an educational establishment, library, etc., are offered to outsiders

b. (as modifier): a university extension course

5. Logic

a. the class of entities to which a given word correctly applies: thus, the extension of satellite of Mars is the set containing only Deimos and Phobos

b. conservative extension a formal theory that includes among its theorems all the theorems of a given theory

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extension [ik′sten·chən]

(mathematics)

extension fields

(physiology)

A movement which has the effect of straightening a limb.

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1.	(filename extension)	extension - filename extension.
2.	(programming)	extension - A feature or piece of code which extends a program's functionality, e.g. a plug-in.