Zettai Ryouiki (絶対領域, Absolute Territory): The part of the skin visible between the skirt and stockings or socks for female characters. The stockings or socks must be above the knees to count as "true" Zettai Ryouiki.

Here I include some common failure modes, and a character showing 2 failure modes is disqualified.
Zettai Ryouiki (Absolute Territory) tier list as of 6.3