territory
n. C / Un. an area of land that belongs to a specific country, person, or animal. You can also use it to talk about a subject or field of knowledge that someone knows well.
n. a geographic area under the jurisdiction of a state or ruler; alternatively, an area defended by an animal or group against intruders. Often used figuratively to describe a specific sphere of activity or knowledge.
The birds defend their territory from other species during the spring.
The disputed territory along the border has been a source of tension between the two nations for decades.
While the marketing department usually handles social media, the new digital strategy falls squarely into the IT department's territory, leading to significant internal friction over budget allocation.
Often used with the preposition 'in' when referring to abstract fields of knowledge or 'over' when discussing control of land.