Pequeño programa que añade alguna función a otro programa, habitualmente de mayor tamaño. Un programa puede tener uno o más "plug-ins".

A file containing data used to alter, enhance, or extend the operation of a parent application program. The Netscape Navigator World-Wide Web browser supports plug-ins which display or interpret a particular file format or protocol such as Shockwave, RealAudio, Adobe Systems, Inc. PDF, Corel CMX (vector graphics). The file to be displayed is included in a web page using an EMBED HTML tag.