A popover is a container-type graphical control element that hovers over its parent window. It can contain various other graphical control elements such as checkboxes, radio buttons, or list boxes. Like any container-type graphical control element, it is meant to group elements that belong together.
Popover graphical control elements were introduced in GTK+ 3.12.
Apple included popovers in their human interface guidelines. Popovers are in use on the web. Bootstrap has a component to create popovers, similar to those found in iOS.
When hovering a link in Wikipedia, by default, there is a popup of page preview that is in a way popover: it includes a button and it has more content than in tooltip.
References
"Popovers in GTK+". 2014-01-22. https://blogs.gnome.org/mclasen/2014/01/22/and-now-popovers/ ↩
Popovers, developer.apple.com https://developer.apple.com/design/human-interface-guidelines/components/presentation/popovers/ ↩
Popovers https://getbootstrap.com/docs/5.0/components/popovers/ ↩