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Software component that adds a specific feature to an existing software application

In computing, a plug in (or plugin, add in, addin, add on, or addon) is a software component that adds a specific feature to an existing computer program. When a program supports plug ins, it enables customization. A theme or skin is a preset package containing additional or changed graphical appearance details, achieved by the use of a graphical user interface (GUI) that can be applied to specific software and websites to suit the purpose, topic, or tastes of different users to customize the look and feel of a piece of computer software or an operating system front end GUI (and window managers).

The host application provides services which the plug in can use, including a way for plug ins to register themselves with the host application and a protocol for the exchange of data with plug ins. Plug ins depend on the services provided by the host application and do not usually work by themselves. Conversely, the host application operates independently of the plug ins, making it possible for end users to add and update plug ins dynamically without needing to make changes to the host application. Programmers typically implement plug ins as shared libraries, which get dynamically loaded at run time. HyperCard supported a similar facility, but more commonly included the plug in code in the HyperCard documents (called stacks) themselves. Thus the HyperCard stack became a self contained application in its own right, distributable as a single entity that end users could run without the need for additional installation steps. Programs may also implement plug ins by loading a directory of simple script files written in a scripting language like Python or Lua.

In Mozilla Foundation definitions, the words "add on", "extension" and "plug in" are not synonyms. "Add on" can refer to anything that extends the functions of a Mozilla application. Extensions comprise a subtype, albeit the most common and the most powerful one. Mozilla applications come with integrated add on managers that, similar to package managers, install, update and manage extensions. The term, "plug in", however, strictly refers to NPAPI based web content renderers. Mozilla deprecated plug ins for its products. But UXP based applications, like web browsers Pale Moon and Basilisk, keep supporting (NPAPI) plug ins.

A helper application is an external viewer programlike IrfanView or Adobe Readerthat displays content retrieved using a web browser. Unlike a plugin whose full code would be included in the browser's address space, a helper application is a standalone application. Web browsers choose an appropriate helper application based on a file's Media type as indicated by the filename extension.

In the mid 1970s, the EDT text editor ran on the Unisys VS/9 operating system for the UNIVAC Series 90 mainframe computer. It allowed a program to be run from the editor which can access the in memory edit buffer. The plug in executable could call the editor to inspect and change the text. The University of Waterloo Fortran compiler used this to allow interactive compilation of Fortran programs. Early personal computer software with plug in capability included HyperCard and QuarkXPress on the Apple Macintosh, both released in 1987. In 1988, Silicon Beach Software included plug in capability in Digital Darkroom and SuperPaint.