An ad that displays in a new browser window behind the current browser window.
The pop-under ad is the sneakier relative of the pop-up ad. While pop-up ads are often shown (and closed) instantly, pop-under ads linger behind the current browser window, appearing only after other windows have been closed.
jenett.org : ASAP!
"A Stand Against Pop-under ads!"
Making Pop-Under Ads Work?
ClickZ (July 15, 2002)
Online Ads That Are Irksome–and Effective
BusinessWeek Online (September 3, 2001)
Yahoo Puts Pop-Unders to the Test
ZDNet News (July 30, 2001)
How to Make Users Hate Online Advertising
ClickZ (August 2, 2001)
X10: What Counts, Though You Might Not Like It
ClickZ (July 27, 2001)
‘Pop-Under’ Ads Click Off Web Surfers
FOXNews.com (July 6, 2001)
The Bad News About Pop-Under Spam
CNET News.com (June 20, 2001)
The X10 Question: Traffic Without Dollars?
E-Commerce Times (June 14, 2001)
Online Ads Get In Your Face
CNET News.com (June 13, 2001)