a possession: land or property held as one's own, whether granted to a people to occupy or received by hereditary right
This word names the thing that is held, not the act of taking it. It comes from the verb "to take possession of, to dispossess" (yarash), so a possession here is almost always a stretch of land that someone now holds because it was handed to them, most often territory a nation is given to settle and keep.
Two shades run through its dozen uses, and English needs two different words to carry them. In most places it is a granted possession: God assigns Mount Seir to Esau, the land east of the Jordan to Reuben and Gad, or the whole land to Israel, each people receiving a territory to hold as its own. In a few places the accent falls instead on inheritance, a holding passed down and kept within a family or tribe, as when a tribe of Israel must not be wiped out because it needs heirs to carry its portion forward.
The granted sense and the inherited sense are not far apart: a land given by God is also a land meant to stay in the family for good. The word simply leans one way or the other depending on whether the moment is about receiving the territory or about keeping it in the line. The far more common Hebrew word for a family inheritance is נַחֲלָה; this word is rarer and keeps a stronger flavor of land taken over and held.