Source : Free On-Line Dictionary of Computing

plug-in
     
         A file containing data used to alter, enhance, or
        extend the operation of a parent {application program}.  One
        of the first uses of this term was in {Silicon Beach}'s
        {SuperPaint} application (late 1980s?) for the {Macintosh}.
        It had a Plug-ins {folder} containing different tools and
        effects.
     
        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}.
     
        Plug-ins, both commercially and indepently authored, can
        usually be downloaded for free and are stored locally.
        Plug-ins come in different versions specific to particular
        {operating systems} ({Microsoft Windows 3.1}, 3.2, and
        {Macintosh} are available).
     
        Compare {applet}.
     
        {Plug-in Plaza (http://www.browserwatch.com/plug-in.html)}.
     
        {PC Mag Plug-ins
        (http://www.zdnet.com/pcmag/IU/plugins/plugins.htm)}.
     
        (1996-05-25)