Formal:
* Own: Implies legal ownership.
* Hold: Focuses on the act of having something.
* Control: Highlights power over something.
* Have: General and neutral, but may not convey the same sense of ownership as "possess."
Informal:
* Have: A common synonym in everyday speech.
* Own: Similar to the formal use, but more casual.
* Carry: Implies physically having something on one's person.
* Stash: Suggests hiding or concealing something.
More specific:
* Wield: Specifically implies power or control over something.
* Grasp: Focuses on understanding or holding something mentally.
* Encompass: Implies holding something within something else.
The best synonym depends on the specific situation and the meaning you want to convey.