territory; domain; sphere of influence
Refers to a person's or group's area of control, expertise, or activity. Often used in contexts like 領分を侵す (invade someone's domain) or 領分を広げる (expand one's territory).
That's not my area of expertise, so I'll leave it to a specialist.
He was desperate to protect his own territory.
Compound of 領 (territory, control) and 分 (portion, share). The exact historical derivation is uncertain, but the combination naturally yields the meaning of one's allotted area of control or influence.