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.
[1] Apple included popovers in their human interface guidelines.
Bootstrap has a component to create popovers, similar to those found in iOS.
[3] 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.