Literally meaning "Absolute Territory", zettai ryouiki is the area of uncovered thigh between the skirt and the knee sock.
Zettai Ryouiki (絶対領域) literally translates to “absolute territory”.
It was originally used in Neon Genesis Evangelion to mean "Absolute Terror Field" (AT Field), but somewhere along the line through memetic mutation, zettai ryouiki has come to mean the area of uncovered thigh between the skirt and the knee sock as seen on many popular female characters. It has become known as a popular moe trait in Japan, and there is even a "golden ratio" of sock to exposed skin to skirt length on the upper thigh used to judge a characters zettai ryouiki by. The golden ratio of the perfect zettai ryouiki is 4 : 1 : 2.5 (length of skirt : zettai ryouiki : length of socks above knee) and the acceptable margin of error is ±15％.
That said, now there is even a grading scale judge by, which is as follows:
and ankle socks are an instant Grade F. Only the top two (A and B) are considered true zettai ryouiki and they are only properly achievable by female characters.
There is also a Grade S that can be reached from Grade A with the addition of certain other factors: