插件（又译外挂，英文为Plug-in、Plugin、add-in、addin、add-on、addon或extension）是一种电脑进程，透过和应用进程（例如网页浏览器，电子邮件用户端）的互动，用来替应用程序增加一些所需要的特定的功能。最常见的有游戏、网页浏览器的插件和媒体播放器的插件。

应用进程之所以支持插件的使用原因很多，主要包括：使得第三方的开发者可以对应用进程进行扩充、精简，或者将源代码从应用进程中分离出来，去除因软件使用权限而产生的不兼容。

In computing, a plug-in (or plugin, add-in, addin, add-on, addon, or extension) is a software component that adds a specific feature to an existing computer program. When a program supports plug-ins, it enables customization. The common examples are the plug-ins used in web browsers to add new features such as search-engines, virus scanners, or the ability to use a new file type such as a new video format. Well-known browser plug-ins include the Adobe Flash Player, the QuickTime Player, and the Java plug-in, which can launch a user-activated Java applet on a web page to its execution on a local Java virtual machine.