A pop-under is also referred to as a pop-under ad.

See "How to Deal With Pop-Ups" in the "Did You Know..." section of Webopedia.

Webopedia Quick Reference: Common Windows PC Issues
Tips to Diagnose and fix a slow running computer.

Webopedia's "Did You Know...?" Section
Use this Webopedia knowledge section for an in-depth overview of specific technologies and occurrences in the areas of Computer Science, The Internet, and Computer Hardware and Software.

Webopedia's Quick Reference Section
Use this Webopedia reference section for information on common Internet and computer facts and occurrences.