Pop-under

Automatically launched internet advertisement that appears in a small window behind another webpage. See ‘Pop-up'.
Found on http://www.cim.co.uk/resources/glossary/home.xhtml?letter=m

Pop-under

Automatically launched internet advertisement appearing behind another webpage.
Found on http://www.chriswallcreative.co.uk/glossary.htm

Pop-Under

On a computer, an advertisement, etc., which comes up on the screen behind the web page which is being viewed, and does not appear until the page is closed
Found on http://www.businessballs.com/business-dictionary.htm
No exact match found