Data's head.
The head is the uppermost part of a humanoid, it contains the brain, skull, eyes, ears, nose and mouth.
Data's head was once left in the past under the city of San Francisco for more than 500 years. (TNG: "Time's Arrow").
- Head can also refer to a bathroom. This was the term used in Star Trek: The Next Generation USS Enterprise NCC-1701-D Blueprints.