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). To qualify for the tag, the skirt should fall naturally down the leg, if the skirt is pulled up or otherwise out of place, the image probably doesn't qualify for the tag. Example characters include Tohsaka Rin, Momose Kurumi and Mayura.
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.
For reference see post #933691.
The tag is not viable if the image shows panties in between the thighhighs and skirt.
Examples of what does not meet the tag criteria:
This tag implicates thighhighs.