There should be the possibility to search for words definitions using Wiktionary.
Maybe there is an API that could be used for that.
Wiktionary supports multiple languages and maybe, if it gets more used, it will be more improved (maintained) in the future.
The last time I tried to search for word definitions in MS Word 2016, it searched also on an on-line dictionary (from Bing?).
Makes sense as an extension.
Seems like what i've suggested with the research sidebar deck/tab (bug 98372).
I wrote some sample code to fetch definitions from Wiktionary, and then filter it via Beautiful Soup to be shorter and more relevant:
If I figure out how to put it into a popup HTML dialog box, it could be reasonable.
Let's make this a duplicate.
*** This bug has been marked as a duplicate of bug 98372 ***