Adding definition popups/tooltips into the editor would be incredibly useful for articles, for example, I have a Ghost site for political articles in Aotearoa/New Zealand, and in NZ we use quite a few words of te reo Māori (our indigenous language) in some writing, and it can be useful to add tooltips to explain these words to readers who may not understand, such as less-aware domestic and foreign readers.
This is effectively a suggestion based off of this post: https://forum.ghost.org/t/adding-definition-popups-in-ghost-articles/48823.
In that, it uses tooltips for other concepts that may not necessarily be widely-understood by everyone, beyond the example I provided, such as LGBTQ+ terms.