See Also: plug-in.
GNU Image Manipulation Program
See: procedural database
From the Free On-line Dictionary of Computing:
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.
Where GIMP stores the list of available procedures.