You can add tooltips to your content. These tooltips work well for glossary definitions.
Because this theme is built on Bootstrap, you can simply use a specific attribute on an element to insert a tooltip.
Suppose you have a glossary.yml file inside your _data folder. You could pull in that glossary definition like this:
<a href="#" data-toggle="tooltip" data-original-title="{{site.data.glossary.jekyll_platform}}">Jekyll</a> is my favorite tool for building websites.</a>
This renders to the following:
Jekyll is my favorite tool for building websites.