Overlays & Surfaces
Popup
also known as pop-up, pop up
Popup is an everyday, imprecise term for anything that suddenly appears over the page: dialogs, popovers, newsletter overlays, even browser-spawned windows. Because it's vague, designers usually prefer a specific term (modal, popover, tooltip). Historically 'pop-up' also meant a whole new browser window opened by a site.
Don't confuse it with
'Popup' is casual and ambiguous, covering many components. 'Popover' is a specific, defined component anchored to a trigger. When precision matters, name the actual component (modal, popover, tooltip) rather than 'popup'.
In the wild
The 'Subscribe to our newsletter' box that appears on a blog is commonly called a popup.