Territory is a term that was introduced in the Chapter Black Saga.
Territory is a power used by most psychics. Once someone's territory is active it enables their psychic abilities to work only within the area of their territory. Anyone caught in someone's territory when it was activated falls subject to the psychics territorial powers and follows the rules of the territory.
The first time the term is mentioned (in the Funimation English Dub of the Anime) is during the Chapter Black Saga by Yu Kaito.