A plugin is a piece of software application consisting of a group of functions that can be contributed to a WordPress website.
https://www.storeboard.com/blogs/general/a-plugin-is-a-piece-of-software-application-including-a-group-of-functions-that-can-be-added-to-a-wordpress-website/1801037
A plugin is a piece of software application containing a group of functions that can be contributed to a WordPress site.