Zettai ryouiki (絶対領域, lit. "absolute territory") is the area of bare thighs seen between a skirt and thighhighs. The skirt should not be lifted up or out of place; the thighs should be exposed naturally, by the skirt being too short to cover the tops of the thighhighs. Example characters include Tohsaka Rin, Momose Kurumi and Mayura.
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.