A plugin is a piece of software application containing a group of functions that can be added to a WordPress site.
https://marachkgfw.doodlekit.com/blog/entry/7108052/a-plugin-is-a-piece-of-software-containing-a-group-of-functions-that-can-be-added-to-a-wordpress-site
A plugin is a piece of software application including a group of functions that can be contributed to a WordPress website.