That could have many different meanings. In no particular order, here's some I can think of...
Real places...
- The country/county/town etc. which a person identifies as their home town, homeland or nation.
- An area of town where their community or family which they belong to lives.
- An area which a gang they belong to claims as its own.
- An area that they cover as part of their job (e.g. delivery driver).
- Land or property that they own or make their money from (e.g. farmer or landlord).
- The area where they know the streets well enough to never feel lost or out of place.
- The place that is their private home, even if it's not actually owned by them (i.e. rented or leased.)
- A particular room in their home where they can get privacy from the other people who live there.
- Inside their car, at their allotment, favourite fishing spot, etc...
- An area around their body far enough from other people so that they don't feel frightened or uncomfortably intimate.
Metaphorically...
- An area of knowledge or skill in which they think they are very accomplished (e.g. "Ah, a science question, just my territory.")
- The parts of their life and thoughts that they consider private rather than public.
- Other people who they feel they have the right to dominate (e.g boss -> workers, gang leader -> gang members.)
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