A Directional antenna
Short for Biquadratic
More Info Links
http://trevormarshall.com/biquad.htm - biquad overview and theory (11dBi)
http://martybugs.net/wireless/biquad/ - detailed biquad construction info (11dBi)
http://martybugs.net/wireless/biquad/double.cgi - a double biquad (13dBi)