Final answer:
The perception of head orientation when the body is stationary is called static equilibrium, which is sensed by the vestibular system detecting gravity and acceleration.
Step-by-step explanation:
The term that refers to the perception of the orientation of the head when the body is stationary is static equilibrium. Our vestibular system is responsible for sensing changes in head position through the detection of gravity, linear acceleration, and angular acceleration and deceleration. When our body is in a state of static equilibrium, it means that we are not experiencing any linear or rotational acceleration in our selected frame of reference, which could be perceived as a sense of balance or stability when the head is positioned upright.