Zettai ryouiki (絶対領域) loosely translates into "absolute territory" and is the area of bare skin seen between a skirt and thighhighs. Some have suggested that the ideal ratio is 4:1:2.5 (length of mini-skirt : absolute territory : length of overknee socks above the knee). Example characters include Tohsaka_Rin, Hatsune_Miku and Shana.
The term itself originates from Neon Genesis Evangelion, although it had none of the current connotations in the anime. The modern usage of zettai ryouiki originates from the creator of a desktop mascot application when he used the term on his blog in reference to Mayura.
Bear in mind that if the girl's panties are visible the tag cannot be used as the "zone" is no longer absolute.