it's basically the space of skin between a girl's miniskirt and higher-than-the-knee thighs.
coming from the japanese phrase, "絶対領域" (zettai ryoiki), it was first a slang coined up by otakus.
now it's used in japan pretty commonly by the public.

it's also a way to tease guys, since they normally like that more that full nudity.

a good example of where this term is used is in the anime, ナナマル サンバツ (nana maru sau batsu), meaning "fastest finger first," by character mari fukami (深見 真理).
*testing shiki on random quiz terms*
mari : "then.. how about questions like this..? here/"
shiki : "uee?~"
mari : "Q: the are of exposed skin between the hem of a miniskirt and the tops of knee-high socks is known by what name?"
shiki : t-there's a name for that?
mari : "as i suspected, you like books, but you're not an otaku."
shiki : "wh-what's the answer?"
mari : "abso.. lute.... terr.. ito.. ry."

*and starts to explain how it's connected to moe culture.*
*shiki start's entering absolute territory into his dictionary device*
mari : "you won't find it."
by your neighborhood weeb July 29, 2017
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The area between shorts/skirts and stockings that expose part of a female's thighs.

It generally attracts more attention than cleavage, mostly the attention of otakus. Mostly done and looks infinitely better on lolis than any other female.

The attention given to the Absolute Territory is automatic. People will ignore your cleavage if you have this.
Guy1: Whoa! Check out that chick's Absolute Territory!

Guy2: She may be showing a shitload of cleavage, but I can't take my eyes off that Absolute Territory!
by That one guy in some place December 29, 2011
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