What's this type of content actually called? I think I've been getting the terminology wrong.

When I open a Panel and go to the Content region and click the gear icon, then Add Content a modal appears.

On the left of this modal are categories housing various site components - gallerys, forms, HTML, etc

What are these things collectively actually called?
I think those are called site content (but I don't know if you'll find a name for all of them written somewhere, as you said).