Zettai Ryouiki

Zettai Ryouiki (絶対領域) is a term used in anime series for the area of exposed thigh when a girl is wearing a miniskirt and thigh high socks. It literally means "Absolute Territory" in Japanese.

Etymology
The term comes from the anime series Neon Genesis Evangelion. To be exact, the term originated from the Absolute Terror Field (AT Field) which powers the Eva units.

As a Moe Attribute
Zettai ryouiki is usually present in characters with the tsundere archetype, like Tohsaka Rin from Fate/stay night, from Zero no Tsukaima, and  from Shakugan no Shana.

The presence of the zettai ryouiki, however, does not exclude non-tsundere characters. Also, it is a moe attribute, alongside megane, nekomimi, and tsundere mentioned above.