The Two Meanings of "Absolute Territory" :
1. A phrase that describes an area of exposed thigh when a girl is wearing a miniskirt and thigh high socks. A fetish most otakus loved.
2. A direct Japanese translation for Absolute Terror Field from Neon Genesis Evangelion (Short form AT Field) will result in 絶対恐怖領域 (Zettai Kyoufu Ryouiki), which is Zettai Ryouiki after shortened.
For more about AT Field, See also : Neon Genesis Evangelion
Origin
The origin of Absolute Territory can be traced back to 2001, where an A.I based desktop resident chatterbot software, Jinkou Munou was circulating around that time. The creator of the character Seriko in the software, MIY announced his next character release for the software in his blog, which is Mayura. Mayura's costume is designed to be completely black from the top to the bottom with greytone check and miniskirt along with black overknee socks and boots. During the announcement, he spoke of this line :
"The distance between kneesocks and mini skirt is invicible! I can even say that it's the God's Absolute Territory..."
After 5 days, he explained the concept of Absolute Territory even further with the illustrations released on his blog, and thus the term spreaded from there.