Zettai ryouiki (絶対領域) loosely translates into "absolute territory" and is the area of bare skin on the thighs seen between a skirt or dress and thighhighs. It may also apply to characters wearing shorts, hotpants and spats. It generally doesn't apply for characters wearing sukumizu, which usually lack any extension down the thigh. 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, Aisaka_Taiga and Mayura.
Panchira images may be eligible for this tag. Where pantsu are incidentally visible, rather than a focus of the artwork, zettai ryouiki may be added if it meets the description above. If the camera angle is so low that the post is viewed_from_below, or if a skirt_lift or similar is involved, the post is less likely to classify as zettai ryouiki.
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.
- AnimeSuki: http://forums.animesuki.com/showthread.php?t=29911
- Ramblings of DarkMirage: http://www.darkmirage.com/2006/12/15/japanese-titbits-3-zettai-ryouiki/