Plugin

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(Redirected from Plug-in)

A plugin (or plug-in) is a computer program that can, or must, interact with another program to provide a certain, usually very specific, function. Typical examples are plugins to display specific graphic formats (e.g., SVG if the program doesn't support this format natively), to play multimedia files, to encrypt/decrypt email (e.g., PGP), or to filter images in graphic programs. The main program (a web browser or an email client, for example) provides a way for plugins to register themselves with the program, and a protocol by which data is exchanged with plugins.

Plugins are slightly different from extensions, which modify or add to existing functionality. The main difference is that plugins generally run within a sandbox, rely on the main program's user interface, and have a well-defined boundary to their possible set of actions. Extensions generally have less restrictions on their actions, and may provide their own user interfaces. They are sometimes used to decrease the size of the main program and offer optional functions. Mozilla Firefox uses a well-developed extension system to reduce the feature creep that plagued the Mozilla Application Suite.

Perhaps the first applications to include a plugin function were HyperCard and QuarkXPress on the Macintosh, both released in 1987. In 1988, Silicon Beach Software included plugin functionality in Digital Darkroom and SuperPaint, and the term plug-in was coined by Ed Bomke. These days, plugins are typically implemented as shared libraries which need to be installed in a place prescribed by the application. HyperCard supported a similar facility, but it was more common for the plugin code to be included in the HyperCard documents (called stacks) themselves. This way, the HyperCard stack became a self-contained application in its own right, which could be distributed as a single entity that could be run by the user without the need for additional installation steps.

Open Application programming interfaces (APIs) provide a standard interface, allowing third parties to create plugins that interact with the main application. A stable API allows third-party plugins to function as the original version changes and to extend the lifecycle of obsolete applications. The Adobe Photoshop and After Effects plugin APIs have become a standard and been adopted to some extent by competing applications. Other examples of such APIs include Audio Units and VST.

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