From Longman Dictionary of Contemporary Englishpop-underˈpop-under noun [countable] a window containing an advertisement, that suddenly appears under another window on a computer screen when you are looking at a website → pop-upFrom Longman Business Dictionarypop-underˈpop-under noun [countable]COMPUTINGMARKETING a small window containing an advertisement that suddenly appears under another window on a computer screen when you are looking at a website
