Drupal
The enterprise OpenSource content management system which is available for free at https://www.drupal.org where you can find all the details and also references.
Drupal has a very active community with tens of thousands of developers all around the world and they also have a huge focus on standard compliance and on security.
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.
The first meeting of the Drupal User Group Bodensee (Lake Constance) in 2017 will be about migration to Drupal 8.
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.