Since custom code can be placed inside (embedded) a rich text field you could accomplish this with custom code. All the content would need to be HTML and normally one would use a script that targeted data attributes.
Thanks @webdev. I guess it might be tricky to do in large volumes (e.g. 50+ words/definitions per CMS item).
Or perhaps for each CMS item there could be a link to an external JSON file with the words/definitions, then there was a script that looped through for each word on the page and if it existed in the JSON file, a tooltip could be created.
@anthonysalamin this could be achieved without specifying which words in the CMS Rich Text Element were keywords, right? The text that contains the words (to be defined) will be circa 2K words, and there will be approximately 100 words to be defined per CMS item.
Thanks for your swift response btw.
Given the volume of words, and the fact that a different definition might be given in a different CMS item (to reflect the fact that words mean different things in different context), I think this could end up being very wieldy.