A plugin is a piece of software application including a group of functions that can be added to a WordPress site.

https://herianuqsh.doodlekit.com/blog/entry/7108810/a-plugin-is-a-piece-of-software-application-containing-a-group-of-functions-that-can-be-added-to-a-wordpress-site

A plugin is a piece of software application consisting of a group of functions that can be added to a WordPress website.