intrusive new synonyms pop-up
Immediately upon restarting, after updating to macOS 15.4.1 this morning, I now have a new pop-up that either (a) reports 'Nothing found' or (b) presents a list of synonyms, for any editable word I double-click on or adjacent to. I attached screenshots of a test in TextEditor.app and here in the 'Provide more details' box of this support question editor.
The same occurs in every app, including browsers, in which there is editable text.
I have searched all of my third-party utilities I am aware of, and have searched Activity Monitor for processes that appear possibly relevant, finding nothing.
I also went through all Systems Settings I thought might be relevant and found nothing.
What is this pop-up and how can I disable it?
MacBook Pro 16″