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An add-in is a software program that expands the capabilities of bigger programs. It is a term commonly used by Microsoft and other platforms which have additional functions that can be added to primary programs. An add-in has specific but limited features that require only minimal memory resources.
An add-in cannot run by itself and is designed to be used with a particular program which is installed separately. Once the add-in is installed, it becomes part of the larger program.
An add-in should not be confused with an add-on, which is a hardware term for an expansion unit.
Some software programs come with add-ins, but most add-ins are sold separately. ArcGIS is an example of a very large database that has several different add-ins designed to extend the application for custom performance.
Not all software programs refer to the additional features for a program as add-ins; Dreamweaver provides "extensions" for its added Web development features, and many graphics and audio programs support "plug-ins".
Popular applications that use add-ins are Microsoft's Outlook, Excel, Word, Visual Studio and Expression Web, various Adobe programs, ArcGIS, and several Macintosh applications. For example, according to the Microsoft Style Guide, the Analysis Toolpak is an add-in for Excel, whereas Microsoft Bookshelf is an add-in for Word.