A type of window that appears on top of (over) the browser window of a Web site that a user has visited. In contrast to a pop-under ad, which appears behind (in back of) the browser window, a pop-up is more obtrusive as it covers other windows, particularly the window that the user is trying to read. Pop-ups ads are used extensively in advertising on the Web, though advertising is not the only application for pop-up windows.
A pop-up ad is also referred to as a pop-up.
Also see pop-up window and pop-up utility.