THE TAG IS NOT VIABLE IF THE IMAGE SHOWS PANTIES/SIMILAR UNDERWEAR OR GENITALIA BETWEEN THE THIGHHIGHS AND SKIRT.
Zettai ryouiki (絶対領域, lit. "absolute territory") is the area of bare skin seen between a skirt and thighhighs. To qualify for the tag, the skirt should fall naturally down the leg, if the skirt is lifted up or otherwise out of place, the image probably doesn't qualify for the tag. Example characters include Toosaka 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.