What's known as plugin or addon in other platforms, is what we call a in the 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.
The User hash allows you to create an individual hash for each user. Use it as a light-weight user identification where you do not want to use the login credentials. E.g. as an individual API Key for reading insensitive content.