I am happy to mention that there is now a solution for creating a glossary on this forum!
With help of Discourse support team I have installed a Theme Component that allows Admins to add words and abbreviations plus an additional text, using a limited (in functionality) UI:
The component is really meant for frequently occurring abbreviations, and therefore will offer limited, not extensive, glossary support. The terms that are defined in the UI will automatically lead to a dotted line to be added to the words when they are used in a post. When you hover over the term, the explainer text will be displayed.
Let’s show this in action in sentence:
“The Cognitive Bias explainer was really tl;dr for me, so I should rtfm later.”, said the man.
Note: Terms can be suggested by adding to the wiki post here: Glossary of Common Terminology