WordNet (r) 3.0 (2006):

plug-in
    n 1: a printed circuit that can be inserted into expansion slots
         in a computer to increase the computer's capabilities [syn:
         circuit board, circuit card, board, card, plug-
         in, add-in]

The Free On-line Dictionary of Computing (30 December 2018):

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 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://browserwatch.com/plug-in.html).

   PC Mag Plug-ins
   (http://zdnet.com/pcmag/IU/plugins/plugins.htm).

   (1996-05-25)