Almost all websites have one requirement in common: getting notified when something specific is happening. Whether that's a new blog post, a new comment, a modified version of a node, an exception in the logs or a deleted user account, either the site admin or website visitors would like to know about some or all of these.
Module
What's known as plugin or addon in other platforms, is what we call a module in the Drupal universe. Drupal core itself is a a collection of modules as well and the community has added and maintains tens of thousands of individual modules on top of that.
The Drupal 8 module Onomasticon automatically displays the definition of taxonomy terms everywhere a term is used. However, you may want to avoid self-definition on the term's own page as its description is already visible without it.