A window that pops up on the screen suddenly, often without warning, advertising something. Nobody ever actually buys what the pop-up is advertising. They're just annoying pieces of crap. Some people create ones that install malware or other unwanted files onto a user's computer.
I clicked the download button for a file on Megaupload and a pop-up ad came up advertising a free iPad. I knew it had to be a scam, so I tried to close it, but it wouldn't close. So I had to close Firefox in Task Manager, but that closed ALL running Firefox processes, not just the pop-up ad. Freakin' annoying.