Possession

From Ultipedia

Typically possession of the disc is established once the disc is in hand and is not spinning.

From the UPA_Rules_of_Ultimate (10th ed.):
Possession of the disc is sustained contact with, and control of, a non-spinning disc.

1. To catch a pass is equivalent to establishing possession of that pass.
2. Loss of possession due to ground contact related to a pass reception negates that player's possession up to that point.
3. A disc in the possession of a player is considered part of that player.
4. The team whose player is in possession or whose players may pick up the disc is considered the team in possession.