A plugin is a piece of software consisting of a group of functions that can be added to a WordPress site.
http://damienknxd804.jigsy.com/entries/general/a-plugin-is-a-piece-of-software-application--consisting-of-a-group-of-functions-that-can-be-added-to-a-wordpress--site-
A plugin is a piece of software application containing a group of functions that can be added to a WordPress site.