A Web site ad that opens in a separate browser window hidden under the current browser window.
"Tilting toward the obnoxious is a new format dubbed the 'pop-under' that hides behind your Web browser's main window and surprises you when you try to close the browser. Pop-under ads hawking a 'Tiny Wireless Video Camera' were shown so many times last month that X10.com, the camera maker, was the fifth most heavily trafficked site on the Web, according to Jupiter Media Metrix."
Leslie Walker, "Online Advertisers Tinker to Find Gimmicks That Sell," The Washington Post, June 21, 2001
"If you spend much time at all on the Web, you've probably encountered pop-under ads for Seattle-based X10's $ 80 wireless video cameras.
The ads, appearing on Web sites of major media powers such as the Tribune Company and New York Times, are a simple twist on pop-up ads, which have been around for years. Instead of popping up in front of a Web page, pop-unders go behind it."
Martin J. Moylan, "New Pop-Under Web Advertising Earns Attention," Saint Paul Pioneer Press, June 1, 2001
It's not often that a newly minted word becomes wildly popular in a very short time, but that's what has happened with pop-under. In recent weeks, this phrase has garnered dozens of citations in major media including not only the Washington Post (above), but also The New York Times, USA Today, Associated Press, and National Public Radio. This is despite the fact that the phrase appears to be only about a month old, judging by the earliest citation.
Note that the earliest citation not only displays both the adjective and noun forms of the phrase, but also explains that it's based on the much more common phrase pop-up.
Note, too, that you can (at least for now) use the following address to prevent the X10 pop-unders from appearing: